Trimester by Trimester
Due Date Calculation: Naegele's Rule vs. Ultrasound Dating
Your due date is a carefully derived estimate — not a deadline. Here's exactly how clinicians calculate it, why fewer than 5% of babies arrive on that day, and what first-trimester ultrasound changes when LMP isn't reliable.
Clinically reviewed · June 2026
Your due date is calculated by adding 280 days to the first day of your last menstrual period (Naegele's Rule), then refined — if needed — by a first-trimester ultrasound measuring crown-rump length, which is accurate to within five to seven days. Fewer than 5% of babies arrive on the exact date.
The estimated due date (EDD) is among the most consequential numbers in your prenatal record. It anchors decisions about when to offer induction, how to interpret growth scans, and when a pregnancy is considered post-term. Yet it is — and has always been — an estimate, not a guarantee. Understanding exactly how it is derived, and where each method carries uncertainty, helps you hold that number with appropriate perspective.
This article is general health information, not medical advice. Always discuss your specific due date and any revisions with your midwife or OB-GYN.
How Is a Due Date Calculated? Naegele's Rule Explained
The formula most providers and every online calculator use was codified in 1806 by German obstetrician Franz Karl Naegele (1778–1851), drawing on earlier observations by Dutch physician Herman Boerhaave. The arithmetic is simple: take the first day of your last menstrual period (LMP), add seven days, subtract three months, and advance one year. That equals adding exactly 280 days — or 40 weeks — to the LMP date.
Those 280 days break down as follows: roughly 14 days from LMP to ovulation and conception (in a standard 28-day cycle), then 266 days of fetal development. Gestational age in clinical practice is always counted from LMP, which is why a pregnancy is described as "40 weeks" even though actual fetal age from fertilization is closer to 38 weeks.
Online due-date calculators on sites like BabyCenter and The Bump implement exactly this formula. More sophisticated variants allow you to enter your average cycle length — adjusting the assumed ovulation day beyond the default day 14 — effectively producing a modified Naegele estimate. Some accept a known conception date and add 266 days instead. What none of them do is measure your baby directly.
Naegele's Rule presupposes a regular 28-day cycle with ovulation on day 14. Women with shorter, longer, or irregular cycles — including those who have recently stopped hormonal contraception — may have ovulated significantly earlier or later, shifting the true EDD by days to weeks. Research indicates only about half of women can accurately recall their LMP date — the single data point the formula depends on entirely.
How Does First-Trimester Ultrasound Dating Work — and When Does It Override LMP?
Crown-rump length (CRL) measurement by first-trimester ultrasound is the current clinical gold standard for gestational age dating. The crown-rump length is the distance from the top of the baby's head to the base of the buttocks, measured on ultrasound. Because fetal growth in the first trimester is nearly linear and inter-fetal variability is minimal at this stage, size is an accurate proxy for age.
ACOG Committee Opinion No. 700 (originally published May 2017, reaffirmed 2021) specifies that CRL measurement through thirteen weeks and six days is accurate to within five to seven days, and recommends that three separate CRL measurements be obtained and averaged for maximum accuracy. After thirteen weeks and six days, CRL is no longer reliable; providers shift to second-trimester biometry (biparietal diameter, head circumference, abdominal circumference, and femur length), which carries a larger uncertainty window of approximately ten to fourteen days.
The ACOG thresholds for revising the EDD from LMP to ultrasound are:
| Gestational Age at Ultrasound | Dating Method | Revise EDD If Discrepancy Exceeds |
|---|---|---|
| Up to 13 weeks 6 days | Crown-rump length (CRL) | > 7 days |
| 14–15 weeks 6 days | Biometry (BPD, HC, AC, FL) | > 7 days |
| 16–21 weeks 6 days | Biometry | > 10 days |
| 22–27 weeks 6 days | Biometry | > 14 days |
| 28 weeks and beyond | Biometry | > 21 days |
A comparative study published in the Journal of Obstetrics & Gynecology Case Reports found that the average difference between the ultrasound-estimated date and actual delivery was only 0.58 days, versus meaningfully larger discrepancies when relying on LMP alone. A Cochrane Library review similarly concluded that ultrasound dating reduces unnecessary post-term induction by avoiding false "overdue" classifications caused by LMP miscalculation.
For IVF pregnancies, LMP is bypassed entirely. The EDD is calculated directly from the embryo transfer date — making it the most precisely known estimated due date of any conception method.
Why Fewer Than 5% of Babies Arrive on Their Due Date — and What "Full Term" Actually Means
The single most important reframe about due dates is this: the EDD is not a deadline or a prediction. It is the midpoint of a normal distribution of delivery dates.
A retrospective analysis of over one million births in Germany found that 90% of deliveries occurred between 19 days before and 7 days after the expected due date — a span corresponding to roughly 37 to 41 weeks of gestation. The standard deviation of gestational length at delivery is approximately 14 days when gestational age was established by first-trimester ultrasound, and about 16 days when derived from LMP alone. Fewer than 5% of babies arrive on the exact calculated date.
Clinically, the spectrum of normal is divided into four categories under current ACOG guidance:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
Deliveries within the full-term window (39–40 weeks) carry the best outcomes on average, which is why providers do not typically offer or recommend elective early deliveries before 39 weeks in uncomplicated pregnancies. At 41 weeks, the conversation shifts toward discussing membrane sweeping or formal induction — not because the baby is dangerously "late," but because post-term risks begin to rise modestly.
A helpful mental model: think of your EDD as the center of a six-week window (roughly week 37 through week 42) rather than a fixed appointment. Going into that window informed — knowing that an arrival at 38 or 41 weeks is still entirely within the range of normal — reduces anxiety and keeps you focused on the clinical indicators that actually matter: fetal movement, fluid levels, and regular monitoring with your provider.
Special Cases: IVF, Irregular Cycles, and the Clearblue Weeks Estimator
IVF pregnancies. The transfer date is known precisely, eliminating the LMP uncertainty that drives most dating imprecision. A day-5 blastocyst transfer yields an EDD of 261 days (37.28 weeks) from transfer; a day-3 embryo transfer yields 263 days. Your reproductive endocrinologist will calculate this directly and confirm at the first OB ultrasound.
Irregular cycles and recent hormonal contraception. If you have polycystic ovary syndrome, cycles longer than 35 days, or have recently stopped hormonal birth control — which can take two to three cycles to normalize — your LMP-based estimate is especially likely to be inaccurate. ACOG notes that return of regular ovulatory cycles after hormonal contraception can take two to three months or longer. Women who have been charting basal body temperature (BBT) and know their actual ovulation date will often find that their chart-derived conception date aligns closely with what the first-trimester CRL confirms — and differs meaningfully from what the LMP alone predicts.
The Clearblue Weeks Estimator. The Clearblue Digital Pregnancy Test with Weeks Indicator uses two hCG immunoassay strips at different sensitivities to classify results as "1–2 weeks," "2–3 weeks," or "3+" weeks since conception. The device reports 93% accuracy with first morning urine and greater than 90% agreement with 11–13 week CRL ultrasound in peer-reviewed validation. It is a useful orientation before your first ultrasound — but note that the weeks shown reflect time since conception (fertilization), not since LMP. Clinical gestational age adds approximately two weeks. The estimator also loses accuracy after six weeks post-conception when hCG plateaus. Treat it as a helpful early signal, not a substitute for ultrasonographic dating.
Your due date will be confirmed — and revised if needed — at your first prenatal appointment, typically between eight and ten weeks. That single visit, combining a CRL ultrasound with your LMP date and cycle history, produces the most accurate EDD possible and sets the clinical calendar for everything that follows.
Frequently asked
How is a due date calculated from my last period?
Your provider uses Naegele's Rule: take the first day of your last menstrual period (LMP), add seven days, then subtract three months and advance the year. That arithmetic equals adding 280 days — or 40 weeks — to the LMP date. The calculation assumes a regular 28-day cycle with ovulation on day 14. If your cycles run longer or shorter, your provider may adjust the estimate accordingly. ACOG Committee Opinion No. 700 remains the authoritative guidance on this calculation and specifies when a first-trimester ultrasound should override it. Always confirm any calculator-derived date with your midwife or OB at your first prenatal visit.
How accurate is the due date from an ultrasound?
A first-trimester ultrasound measuring crown-rump length (CRL) is the current clinical gold standard and is accurate to within five to seven days through thirteen weeks and six days of gestation. ACOG recommends averaging three separate CRL measurements for maximum accuracy. A comparative study found the average difference between the ultrasound-estimated date and actual delivery was just 0.58 days, far better than LMP alone. After fourteen weeks, second-trimester biometry (biparietal diameter, femur length) is used, carrying a wider uncertainty window of approximately ten to fourteen days. If the ultrasound date differs from your LMP date by more than seven days in the first trimester, your provider will typically revise your EDD to the ultrasound estimate.
Why do so few babies arrive on their due date?
Fewer than 5% of babies are born on their exact estimated due date. This is expected and entirely normal. A large retrospective analysis of over one million births in Germany found that 90% of deliveries occurred between 19 days before and 7 days after the due date — a range corresponding to 37 to 41 weeks of gestation. The standard deviation of gestational length at delivery is approximately 14 days when gestational age was established by first-trimester ultrasound, and about 16 days when derived from LMP alone. Researchers writing in The Conversation note that the EDD is best treated as a reference midpoint within a multi-week window rather than a precise appointment. Clinically, deliveries anywhere between 37 and 42 weeks are generally considered within the normal range absent other concerns.
When should ultrasound override LMP dating?
ACOG Committee Opinion No. 700 specifies clear thresholds. In the first trimester (up to 13 weeks 6 days), if the ultrasound CRL-derived date differs from the LMP date by more than seven days, the EDD should be revised to the ultrasound estimate. In the second trimester (14–27 weeks), the threshold is ten days; in the third trimester (28 weeks and beyond), it is 21 days. Women who have recently stopped hormonal contraception, have irregular cycles, polycystic ovary syndrome, or have had a recent pregnancy loss are especially likely to benefit from ultrasound revision, because their LMP-based estimate carries larger inherent error. IVF pregnancies bypass LMP entirely: the EDD is calculated directly from the embryo transfer date.
How do online due date calculators work?
Calculators on sites like BabyCenter and The Bump implement Naegele's Rule as their default: enter the first day of your LMP and they add 280 days. More advanced variants let you input your average cycle length, which shifts the assumed ovulation day beyond the default day 14. Some calculators accept a known conception date and add 266 days (280 minus 14) instead. None performs CRL measurement or accounts for biological variation beyond cycle length — which is why researchers consistently emphasize confirming any calculator-derived date with your provider and a first-trimester ultrasound. Both BabyCenter and The Bump include this recommendation explicitly in their calculator tools.
Can I use the Clearblue Weeks Indicator as a due date guide?
The Clearblue Digital Pregnancy Test with Weeks Indicator uses two hCG immunoassay strips calibrated at different sensitivities to estimate weeks since conception — not weeks since LMP. Because clinical gestational age counts from LMP, the weeks shown on the Clearblue display are approximately two weeks less than your official gestational age. Clearblue's own data report 93% accuracy when first morning urine is used, and one peer-reviewed study found agreement with 11–13 week CRL ultrasound in greater than 90% of cases. However, the estimator becomes less reliable after six weeks post-conception, when hCG plateaus. Treat a Clearblue weeks result as a useful orientation — not a clinical due date. Clearblue's professional brochure notes explicitly that the weeks indicator does not replace ultrasonographic dating.
Does having an irregular cycle affect my due date accuracy?
Yes — significantly. Naegele's Rule was built for a regular 28-day cycle with ovulation on day 14. Women with cycles shorter than 24 days or longer than 35 days ovulate earlier or later, shifting the true conception date and therefore the accurate EDD. Polycystic ovary syndrome, recent discontinuation of hormonal contraception, postpartum or breastfeeding status, and recent pregnancy loss all introduce additional variability. Research indicates that only approximately half of women can accurately recall the first day of their last menstrual period, the entire foundation of LMP dating. LMP-based dating has a standard deviation of about 16 days in singleton pregnancies. For women with irregular cycles, a first-trimester CRL ultrasound is especially valuable and often results in a revised EDD.