How Due Dates Are Calculated: LMP, Ovulation & Ultrasound Methods

LMP Method (Last Menstrual Period)

The LMP method is the traditional and most commonly used approach to calculate a due date. It is based on the first day of your last menstrual period and has been used by obstetricians for nearly two centuries. According to ACOG Committee Opinion No. 700, the LMP remains the initial method used at most first prenatal visits.

The Formula

Due Date = First day of LMP + 280 days (40 weeks)

This can also be calculated using Naegele's rule: Take your LMP date, add one year, subtract three months, and add seven days. This formula was first published by German obstetrician Franz Naegele in 1830 and remains the foundation of due date calculation worldwide.

Example

If your last menstrual period started on January 1:

• January 1 + 280 days = October 8
• Or using Naegele's rule: Jan 1 + 1 year = Jan 1 next year, - 3 months = October 1, + 7 days = October 8

Assumptions of the LMP Method

This method assumes:

• A 28-day menstrual cycle
• Ovulation on day 14 of the cycle
• Conception around day 14-15

Limitations

The LMP method can be inaccurate if:

• You have irregular cycles
• Your cycle is longer or shorter than 28 days
• You don't remember your exact LMP date
• You were on hormonal birth control recently
• You had breakthrough bleeding that was mistaken for a period

Ovulation Date Method

If you know when you ovulated—through ovulation tracking, OPKs, or fertility treatment—this method can be more accurate than the LMP method. Because it accounts for the actual timing of conception, it eliminates the biggest source of error in LMP-based dating: the assumption that ovulation always occurs on day 14.

The Formula

Due Date = Ovulation date + 266 days (38 weeks)

Since conception occurs around ovulation, and pregnancy from conception to birth averages about 266 days, this method calculates from the actual start of pregnancy. For a deeper look at this calculation, see our guide on calculating your due date from ovulation.

Example

If you ovulated on January 15:

• January 15 + 266 days = October 8

Why It's More Accurate

The ovulation method is often more accurate because:

• It doesn't assume a 28-day cycle
• It accounts for variation in when ovulation actually occurs
• It's based on when pregnancy actually began

How to Know Your Ovulation Date

You might know your ovulation date if you:

• Used ovulation predictor kits (ovulation is typically 24-36 hours after positive test)
• Tracked basal body temperature (ovulation is the day before the temperature rise)
• Had a monitored fertility treatment cycle
• Know the date of IUI or IVF transfer

Our ovulation calculator can help you estimate ovulation based on your cycle length, and if conception occurs, you can use that date to estimate your due date more precisely than LMP alone.

Ultrasound Dating

Ultrasound measurements, especially in the first trimester, are considered the most accurate method for determining gestational age and due date. The American College of Obstetricians and Gynecologists (ACOG) recommends that first-trimester ultrasound is the most accurate method to establish or confirm gestational age.

How It Works

During an ultrasound, the technician measures the embryo or fetus. In early pregnancy, the measurement used is:

• Crown-rump length (CRL): The length from the top of the head to the bottom of the torso, measured between 7-13 weeks

In later pregnancy, other measurements are used:

• Biparietal diameter (BPD): The width of the baby's head
• Femur length (FL): The length of the thighbone
• Head circumference (HC): The measurement around the baby's head
• Abdominal circumference (AC): The measurement around the baby's abdomen

These measurements are compared to standardized growth charts to estimate gestational age.

Ultrasound Dating Accuracy by Trimester

Why Early Ultrasounds Are Most Accurate

In early pregnancy, all embryos grow at nearly the same rate regardless of genetics, making size a reliable indicator of age. As pregnancy progresses, individual variation in fetal size increases due to genetics, nutrition, and other factors, making dating less precise. A study published in the British Journal of Obstetrics and Gynaecology confirmed that early ultrasound provides the most reliable estimate of gestational age.

The Mittendorf-Williams Rule

While Naegele's rule has been the gold standard since 1830, research has shown it may not be entirely accurate for all women. In 1990, researchers Robert Mittendorf and Michelle Williams published a study proposing a revised calculation that accounts for whether a woman is having her first baby or a subsequent pregnancy.

What the Research Found

Mittendorf and Williams studied pregnancy duration in a population of healthy, private-care patients and found that:

• First-time mothers (primipara): Average pregnancy lasted 288 days from LMP (8 days longer than Naegele's 280 days)
• Women with prior pregnancies (multipara): Average pregnancy lasted 283 days from LMP (3 days longer than Naegele's 280 days)

Their findings suggested that Naegele's rule tends to produce due dates that are slightly too early, particularly for first-time mothers.

The Formula

The Mittendorf-Williams rule modifies the traditional calculation:

• First pregnancy: LMP + 1 year - 3 months + 15 days (instead of + 7 days)
• Subsequent pregnancies: LMP + 1 year - 3 months + 10 days

Example

If your LMP was January 1 and this is your first pregnancy:

• January 1 + 1 year = January 1 next year
• - 3 months = October 1
• + 15 days = October 16 (vs. October 8 with Naegele's rule)

Factors the Study Considered

The Mittendorf-Williams research also identified several other factors that can influence pregnancy duration:

• Parity: First pregnancies tend to last longer
• Maternal age: Older mothers may carry slightly longer
• Race and ethnicity: Studies suggest variation across different racial and ethnic groups
• Education and socioeconomic status: Correlations were observed with some demographic factors

Why Some Practitioners Prefer It

Some healthcare providers consider the Mittendorf-Williams rule when:

• A first-time mother consistently measures "behind" by Naegele's dating
• There is no early ultrasound to confirm dating
• They want a more personalized estimate that accounts for parity

However, most providers still use Naegele's rule as a starting point and rely on first-trimester ultrasound to refine the estimate. The NHS in the UK and ACOG in the US both primarily recommend ultrasound-confirmed dating as the most reliable approach.

IVF and Assisted Reproduction Due Dates

For women who conceive through in vitro fertilization (IVF) or other assisted reproductive technologies, due date calculation is often more precise because the exact timing of fertilization and embryo transfer is known. This eliminates the guesswork that comes with estimating ovulation date in natural conception cycles.

How IVF Transfer Dates Work

In IVF, embryos are transferred to the uterus at specific developmental stages. The two most common transfer timings are:

• Day 3 transfer (cleavage stage): The embryo has been developing in the lab for 3 days after fertilization
• Day 5 transfer (blastocyst stage): The embryo has been developing for 5 days and has reached the blastocyst stage

The Formulas

To calculate a due date from an IVF transfer, the embryo age at transfer is subtracted to find the "equivalent ovulation date," then 266 days are added:

• Day 5 (blastocyst) transfer: Due date = Transfer date - 5 days + 266 days = Transfer date + 261 days
• Day 3 (cleavage) transfer: Due date = Transfer date - 3 days + 266 days = Transfer date + 263 days

Alternatively, some clinics calculate an "LMP equivalent" date: for a Day 5 transfer, the equivalent LMP would be the transfer date minus 19 days (5 days for embryo age + 14 days for the pre-ovulation phase of a standard cycle).

Fresh vs. Frozen Embryo Transfer

The calculation is the same regardless of whether the embryo was freshly created or was previously frozen (frozen embryo transfer, or FET). What matters is the embryo age at the time of transfer:

• Fresh Day 5 transfer: Egg retrieval occurred 5 days before transfer
• Frozen Day 5 transfer: Embryo was frozen at the blastocyst stage and thawed for transfer; it is still considered a Day 5 embryo

Known Conception Date Advantage

IVF pregnancies have a significant advantage in due date accuracy because:

• The exact date of fertilization (egg retrieval day) is documented
• The exact developmental stage of the embryo at transfer is known
• There is no uncertainty about when ovulation occurred
• The due date can be calculated to within a day of accuracy, assuming average gestation length

Donor Egg Considerations

When donor eggs are used, the due date calculation remains the same—based on the embryo transfer date and embryo age. However, some research suggests that pregnancies from donor eggs may have slightly different average durations due to immunological factors, though this is not yet reflected in standard clinical guidelines. Your fertility specialist and OB will discuss which dating method they prefer for your specific situation. For more on when to consult a fertility specialist, see our detailed guide.

Comparing All Methods

Which Method Do Doctors Use?

Most healthcare providers follow the ACOG guidelines and will:

1. Start with an LMP-based estimate at your first prenatal visit
2. Confirm or adjust using first-trimester ultrasound
3. If ultrasound differs by more than 5-7 days from LMP date, they may change your due date
4. For IVF pregnancies, use the transfer-based calculation, confirmed by ultrasound

According to Mayo Clinic, the combination of LMP dating confirmed by first-trimester ultrasound provides the most reliable estimated due date for clinical decision-making.

Why Due Dates Are Often Wrong

Despite the best calculation methods available, the vast majority of babies are not born on their estimated due date. Understanding why can help reduce anxiety and set realistic expectations as your pregnancy progresses.

The Statistical Reality

Research consistently shows that pregnancy duration follows a bell-curve distribution, not a single fixed point:

• Only about 5% of babies are born on the exact estimated due date
• About 80% are born within the window of 38 to 42 weeks
• The peak delivery week is week 39-40, with approximately 55% of births occurring during this two-week window
• About 11-12% of births are considered preterm (before 37 weeks), according to March of Dimes

Factors That Affect Birth Timing

First Pregnancy vs. Subsequent Pregnancies

First-time mothers (nulliparous women) tend to deliver an average of 5-7 days later than women who have given birth before. This is one of the reasons the Mittendorf-Williams rule uses different calculations for first vs. subsequent pregnancies. The fertile window timing and the resulting conception date are just the beginning—the body's response to a first pregnancy can differ meaningfully from later ones.

Maternal Age

Some research suggests that women over 35 may have slightly shorter pregnancies on average. However, this effect is small and interacts with other factors like medical interventions, making it difficult to use for individual predictions.

Ethnicity and Genetic Factors

Studies published in peer-reviewed journals have found that average pregnancy duration varies slightly across different ethnic groups. For example, some research indicates that Asian and Black women may have slightly shorter average gestational periods compared to White women. However, these are population-level trends and should not be used to adjust individual due dates without clinical guidance.

Prior Pregnancy History

If you delivered your previous baby early or late, you have a somewhat higher chance of a similar pattern in subsequent pregnancies. A history of preterm birth is one of the strongest predictors of preterm birth in future pregnancies.

Other Contributing Factors

• Body mass index (BMI): Obesity is associated with slightly longer pregnancies and increased risk of post-term pregnancy
• Maternal health conditions: Conditions such as gestational diabetes or preeclampsia can alter timing
• Stress and lifestyle: Chronic stress may influence pregnancy duration
• Fetal sex: Some studies suggest male fetuses may be associated with slightly longer pregnancies

When Your Due Date Gets Changed

It is common for due dates to be adjusted during pregnancy, particularly after the first ultrasound. While this can be emotionally unsettling—especially if you have already mentally locked onto a specific date—understanding why changes happen can provide reassurance.

Reasons for Due Date Adjustment

Irregular Cycles

If your menstrual cycles aren't 28 days, the LMP calculation will be off. A woman with 35-day cycles who had her LMP on January 1 would actually ovulate around January 21 (not January 14), making her true due date about a week later than the LMP calculation suggests.

Late Ovulation

Stress, illness, travel, or other factors can delay ovulation even in women with typically regular cycles. If ovulation occurred later than expected, the pregnancy is "younger" than LMP calculations suggest. Understanding your typical luteal phase length can help clarify these discrepancies.

Early Ultrasound Findings

If the first ultrasound shows the embryo measuring differently than expected based on LMP, your provider may adjust your due date to match the ultrasound findings.

Ultrasound Discrepancy Thresholds

ACOG provides specific guidelines for when ultrasound dating should replace LMP dating:

Early vs. Late Ultrasound Reliability

The reliability of ultrasound dating decreases as pregnancy advances. During the first trimester, embryos grow at a remarkably uniform rate, making CRL measurement extremely precise. By the third trimester, individual genetic variation in fetal size means that two healthy babies of the same gestational age can differ in weight by over a pound, making size-based dating much less reliable.

The Emotional Impact of a Changed Due Date

Having your due date changed—even by just a few days—can feel surprisingly emotional. You may have:

• Already told family and friends a specific date
• Planned maternity leave around the original date
• Felt anxious that something is "wrong" with the pregnancy
• Experienced frustration at the uncertainty

It helps to remember that a due date change almost always reflects better information, not a problem. Think of it as your healthcare team refining their estimate to give you the most accurate timeline possible. What to Expect offers helpful perspective on coping with due date changes during pregnancy.

Due Date Accuracy

Regardless of how it's calculated, a due date is an estimate, not a deadline. The concept of a precise due "date" is somewhat misleading given the natural biological variation in pregnancy duration.

When Do Babies Actually Arrive?

• Only about 5% of babies are born on their due date
• About 80% of babies are born within 2 weeks of the due date (38-42 weeks)
• First-time mothers tend to deliver slightly later on average
• "Full term" is now defined as 39 weeks 0 days through 40 weeks 6 days (not 37 weeks as previously thought)
• "Early term" covers 37 weeks 0 days through 38 weeks 6 days
• "Late term" is 41 weeks 0 days through 41 weeks 6 days
• "Post-term" begins at 42 weeks 0 days

The Due Month Concept

Some healthcare providers, including those at NHS UK, now encourage thinking of a "due month" or "due window" rather than a single due date, to set more realistic expectations. Your baby is considered full term if born anywhere from 39 weeks to 40 weeks 6 days.

Gestational Age vs. Fetal Age: Week-by-Week Reference

Ultrasound Accuracy by Trimester

Ultrasound dating remains the clinical gold standard for confirming gestational age, but its accuracy varies significantly depending on when the scan is performed. According to ACOG Committee Opinion No. 700, first-trimester ultrasound provides the narrowest margin of error and should be used to establish or confirm the due date whenever possible.

The reason accuracy diminishes over time is biological: in early pregnancy, all embryos grow at nearly identical rates regardless of genetics, nutrition, or other individual factors. As the fetus develops, genetic variation in size becomes more pronounced, making it harder to infer gestational age from measurements alone. This is why providers emphasize early dating scans, and why a known ovulation date paired with an early ultrasound produces the most reliable estimate.

Comprehensive Ultrasound Dating Reference

CRL = Crown-Rump Length. BPD = Biparietal Diameter. HC = Head Circumference. FL = Femur Length. AC = Abdominal Circumference.

For the most accurate due date, aim to have a dating ultrasound before 13 weeks. If you are also tracking ovulation with OPKs or BBT charting, share that data with your provider so they can cross-reference multiple dating methods. Research published in the British Journal of Obstetrics and Gynaecology confirms that early ultrasound combined with ovulation data yields the highest confidence in gestational age estimates.

IVF and Assisted Reproduction Due Dates

Pregnancies achieved through in vitro fertilization (IVF) and other assisted reproductive technologies have a unique advantage when it comes to due date accuracy: the exact date of fertilization and the developmental stage of the embryo at transfer are precisely documented. This eliminates the uncertainty that affects natural conception dating, where the ovulation date is often estimated rather than known.

The basic principle is straightforward: because we know the embryo's age at transfer, we can calculate backward to find the equivalent "ovulation date" and then apply the standard 266-day formula. Alternatively, the days are pre-calculated for each transfer type. For more on when to consider fertility treatment, see our guide on when to see a fertility specialist.

IVF Due Date Calculations by Transfer Type

Example Calculation

If your Day 5 blastocyst transfer was performed on March 10:

• Equivalent ovulation date: March 10 - 5 days = March 5
• Due date from ovulation: March 5 + 266 days = November 26
• Or simply: March 10 + 261 days = November 26
• Equivalent LMP date: March 10 - 19 days = February 19 (this is used when entering dates into pregnancy apps or wheels that require an LMP)

IVF due dates are typically confirmed with a first-trimester ultrasound at 6-8 weeks. Because the transfer date is known with certainty, any discrepancy between the IVF-calculated date and the ultrasound measurement can provide valuable clinical information about implantation timing and early growth. According to ACOG, the known timing of IVF makes these pregnancies among the most accurately dated.

When Do Most Babies Actually Arrive?

Despite the emphasis placed on the estimated due date, the reality is that only about 4-5% of babies are born on that exact day. Understanding the actual distribution of delivery timing can help set realistic expectations and reduce anxiety as your due date approaches. Data from March of Dimes and published research provide a clear picture of when babies actually arrive.

Delivery Week Distribution

The following chart shows the approximate percentage of singleton births that occur at each gestational week, based on aggregated data from multiple studies including Mongelli et al. (BMJ 1996) and CDC vital statistics:

Detailed Delivery Timing Reference

What This Means for You

Rather than fixating on a single due date, it is more realistic to think in terms of a "due window." Most healthcare providers and resources like Mayo Clinic encourage expecting parents to prepare for delivery anytime between 37 and 42 weeks, with the highest likelihood falling in weeks 39-40. If this is your first pregnancy, be aware that first-time mothers tend to deliver about 5-7 days later than the statistical average, which means going past your due date is very common and usually not a cause for concern.

For a detailed week-by-week look at what happens as you approach your due date, What to Expect provides an excellent resource covering symptoms, baby development, and preparation tips for each week of pregnancy.

Naegele's Rule vs. Modern Methods

For nearly two centuries, Naegele's rule has served as the default formula for estimating due dates. Yet modern obstetrics now offers several alternative methods, each with distinct strengths and limitations. A 2013 study published in Human Reproduction found that natural variation in pregnancy length is far wider than most expectant parents realize, reinforcing the importance of choosing the right dating method for your circumstances. The UK National Institute for Health and Care Excellence (NICE) recommends first-trimester ultrasound as the primary dating method in its antenatal care guidelines, reflecting a shift away from formula-only approaches across international clinical practice.

Understanding how these five methods compare can help you have a more informed conversation with your healthcare provider about which approach -- or combination of approaches -- is most appropriate for your pregnancy. Each method carries different assumptions, and no single formula works perfectly for every woman. The table below summarizes the key differences.

As the Royal College of Obstetricians and Gynaecologists (RCOG) notes, clinical best practice now favors combining multiple data points rather than relying on any single formula. If your LMP-based date and your ultrasound-based date closely agree, you can be more confident in your estimated due date. If they diverge significantly, your provider will typically adopt the ultrasound measurement as more reliable. For women who have tracked signs of ovulation, sharing that data with your care team provides an additional reference point that can improve dating accuracy.

Why Due Dates Change During Pregnancy

One of the most common sources of anxiety for expectant parents is learning that their due date has been adjusted. This happens more often than most people expect, and understanding why can help reduce unnecessary worry. According to the March of Dimes, only a small fraction of babies arrive precisely on their estimated due date, which underscores the inherent imprecision of all dating methods.

Due dates are revised for several important clinical reasons. The most common scenario occurs when a first-trimester ultrasound measures the baby at a different gestational age than the LMP calculation suggested. This discrepancy typically arises because ovulation did not occur on day 14 of the cycle -- a woman with a 35-day cycle, for example, likely ovulated around day 21, making her LMP-based date roughly a week too early. The Johns Hopkins Medicine resource on pregnancy dating explains that first-trimester ultrasound is the most reliable method for confirming or adjusting a due date.

Common Reasons Your Due Date May Change

• Irregular cycle length: If your cycle is shorter or longer than 28 days, the initial LMP calculation likely overestimated or underestimated gestational age. Women with irregular periods are especially likely to have their dates adjusted.
• Late ovulation: Some women ovulate later than day 14, meaning the baby is younger than the LMP date suggests. Tracking when you ovulate relative to your period can help identify this.
• Uncertain LMP: Breakthrough bleeding, spotting after stopping birth control, or simply not remembering the exact date of your last period can all lead to inaccurate initial estimates.
• Multiple pregnancies: Twins and other multiples often have different growth trajectories that may prompt due date refinement.
• Discrepancy thresholds: ACOG sets specific thresholds -- if first-trimester ultrasound differs from LMP by more than 5 days (before 9 weeks) or 7 days (9-13 weeks), the ultrasound date should be used.

It is important to know that a changed due date does not mean anything is wrong with your pregnancy. It simply means your healthcare provider has obtained more accurate information. Think of your due date as a best estimate that may be refined as new data becomes available -- much like a weather forecast that becomes more precise as the event approaches. If your dates are adjusted, your provider will explain the reason and update all subsequent care milestones accordingly. For women who used our ovulation calculator or tracked their cervical mucus before conceiving, sharing that information can help your provider evaluate whether an adjustment is truly needed.

Full-Term Definitions: Understanding Pregnancy Term Categories

Not all "full-term" pregnancies are created equal. In 2013, ACOG and the Society for Maternal-Fetal Medicine (SMFM) replaced the old binary classification of "term" versus "preterm" with a more nuanced system that recognizes important differences in neonatal outcomes across the final weeks of pregnancy. This reclassification has direct implications for delivery planning, induction decisions, and how you think about your due date. The NICE guidelines on antenatal care similarly emphasize the importance of accurate gestational dating for managing deliveries at the boundaries between these categories.

These definitions matter because outcomes differ meaningfully between a baby born at 37 weeks versus one born at 39 weeks. Research from March of Dimes has shown that babies born even a few weeks early are at higher risk for breathing problems, feeding difficulties, temperature regulation issues, and longer hospital stays. This is why ACOG strongly recommends against elective early-term delivery -- every additional day in the womb during weeks 37-39 supports critical brain and lung development.

For expectant parents, the takeaway is straightforward: rather than focusing on a single due date, plan for a delivery window. If you are a first-time mother, be especially prepared for the possibility that your baby may arrive during the late-term window -- first pregnancies tend to run longer. Discuss your individual risk factors and preferences with your provider well in advance so that you have a clear plan for each scenario. If you conceived through IVF or tracked your ovulation date precisely, the accuracy of your dating gives your provider more confidence in determining when intervention may be appropriate.

Related Reading

Explore our complete library of fertility and pregnancy resources:

• Ovulation Calculator -- Free tool to estimate your fertile window
• About Us -- Learn about our mission and methodology
• The Menstrual Cycle and Ovulation Explained
• Menstrual Cycle Phases Explained
• Signs of Ovulation
• How Many Days After Your Period Do You Ovulate?
• How Long After Ovulation Is Your Period?
• The Fertile Window: When to Conceive
• Basal Body Temperature and Ovulation
• Cervical Mucus and Fertility
• Complete Guide to Ovulation Predictor Kits (OPKs)
• Clearblue Ovulation Calculator Guide
• Best Fertility Tracker Apps Compared
• Irregular Periods and Ovulation
• Ovulation With Irregular Periods
• Due Date Calculator From Ovulation
• When to See a Fertility Specialist

Frequently Asked Questions

Helpful Resources

External Resources

• ACOG: Methods for Estimating the Due Date -- Official clinical guidelines on pregnancy dating
• Mayo Clinic: Prenatal Care -- First trimester visits and what to expect
• March of Dimes: Preterm Birth Data -- National statistics on preterm and term births
• PubMed: Length of Human Pregnancy (Jukic et al., 2013) -- Landmark study on pregnancy duration variability
• PubMed: Mittendorf-Williams Study (1990) -- Original research on revised due date calculation
• What to Expect: Pregnancy Week by Week -- Comprehensive weekly pregnancy guide
• NHS UK: Due Date Calculator -- UK-based due date information and calculator
• NIH/NICHD: Prenatal Care Information -- Evidence-based prenatal care guidance

Related Articles on This Site

• Due Date Calculator from Ovulation -- Calculate your due date using your known ovulation date
• Menstrual Cycle and Ovulation Explained -- Understand the cycle phases that underpin due date calculations
• Irregular Periods and Ovulation -- How cycle irregularity affects dating accuracy
• When to See a Fertility Specialist -- Guidance on IVF and assisted reproduction options