How Do You Work Out Your Due Date?

Most due dates are calculated by counting 280 days (40 weeks) forward from the first day of your last menstrual period, a method credited to the nineteenth-century German obstetrician Franz Naegele. In practice, though, an early ultrasound scan often refines or replaces that initial estimate. The two approaches do not always agree, and the biology underlying both is less precise than the single circled date on your calendar might suggest.

The Standard Calculation From Your Last Period

Naegele’s rule is the starting point for almost every due date. It works like this: take the first day of your last menstrual period (LMP), add seven days, then count forward nine calendar months. The result lands on a date roughly 280 days later, which is where the familiar “40 weeks of pregnancy” figure comes from.¹ Doctors, midwives, and pregnancy apps all use this as a default, even though it bakes in an assumption that may not apply to you: that you ovulated on day 14 of a 28-day cycle.

The rule is simple enough to do on a napkin, but the math is not quite as clean as it looks. A recent analysis found that the calendar-based rule and a straight 280-day count diverge in more than half of all possible start dates across two centuries of the calendar, with every leap-year interval producing a mismatch.² That discrepancy is only a day or two, but it illustrates how even the simplest-seeming formula has hidden quirks. Online calculators and pregnancy apps may use slightly different implementations under the hood, so do not be surprised if two tools give you dates a day apart for the same LMP.

Why Your Cycle Length Changes the Answer

Naegele’s rule assumes ovulation happened 14 days into your cycle. If your cycles run longer than 28 days, you likely ovulated later, which means conception happened later and the baby has had fewer days of actual development than the LMP formula credits. The reverse applies for shorter cycles. A straightforward fix is to adjust by the difference between your cycle length and 28 days. If your cycles are typically 35 days, for instance, you would push the due date seven days later.

This matters more than it might sound. A study of cycle-length categories found that using an unadjusted LMP in women with long cycles dramatically inflated the proportion of pregnancies classified as “post-date” or “post-term.” Among women with the longest cycles, more than half of pregnancies looked overdue on paper, while an adjustment that accounted for later ovulation virtually eliminated the distortion.³ Being falsely labeled post-date can lead to unnecessary interventions like induction, so getting this right has real consequences.

The trouble is that many people do not have a consistent cycle length to report. Cycles vary from month to month, and some people cannot recall their last period with confidence. Irregular cycles, recent use of hormonal contraception, and breastfeeding all make LMP-based dating unreliable, which is why clinicians rarely rely on it alone.

Early Ultrasound Dating

An ultrasound in the first trimester is the most accurate tool for pinning down a due date, and professional guidelines recommend that every pregnancy have one before about 22 weeks to confirm or revise the estimated delivery date.⁴ In the first trimester, the measurement used is crown-rump length (CRL), the distance from the top of the embryo’s head to its bottom. At this stage, embryos grow at nearly the same rate regardless of genetics or nutrition, so size is a reliable proxy for age.

Compared with other early measurements like head circumference or abdominal circumference, CRL produces the smallest prediction errors. One study found the average error in predicting delivery date from CRL was essentially zero, with a standard deviation of about eight days, meaning most predictions fell within roughly a week of the actual delivery.⁵ That is as good as it gets with current technology. Once you move into the second trimester, the baby’s head diameter becomes the go-to measurement, but accuracy drops somewhat. By late in the second trimester, ultrasound dating formulas are less precise still, because individual growth variation starts to widen.⁶

The practical takeaway: get a dating scan early. If your first ultrasound happens at seven weeks, the due date it generates is considerably more trustworthy than one based on an ultrasound at 24 weeks. A dating scan done after 22 weeks is considered “suboptimal” by major obstetric guidelines, and the estimated due date established before that cutoff should generally not be changed later unless circumstances are exceptional.⁴

When LMP and Ultrasound Disagree

It is common for the LMP-based date and the ultrasound-based date to differ by a few days. When the gap is small, clinicians typically stick with whichever method was established first. When the gap is larger, most guidelines favor the ultrasound, particularly when it was performed in the first trimester. The American College of Obstetricians and Gynecologists recommends that the gestational age and estimated due date be determined as soon as data from the LMP, the first accurate ultrasound, or both are available, and that subsequent changes be reserved for rare circumstances.⁷

This “set it and don’t move it” approach exists for a good reason. Changing the due date mid-pregnancy can cascade through other clinical decisions: the timing of genetic screening, the thresholds for growth restriction, and the point at which a pregnancy is considered overdue enough to warrant induction. The estimated due date is the reference point for almost every prenatal test and obstetric intervention, so treating it as a moving target creates confusion and risk.⁷

The Natural Variation Nobody Talks About

Even when you know the exact day of conception, the length of a human pregnancy is not fixed. A study that tracked women who were trying to conceive and confirmed ovulation through daily hormone measurements found that the range of gestation length spanned 37 days among uncomplicated, full-term pregnancies. That is more than five weeks of natural spread, even after excluding preterm births and complications.⁸ The same study found that variation was lower when measured from ovulation than when measured from the last menstrual period, because the LMP method adds extra noise from cycle-to-cycle differences in ovulation timing.⁹

In other words, a five-week window of possible delivery dates is baked into human biology, no matter how precisely you date the pregnancy. Your due date is the center of a bell curve, not a deadline. Only a small percentage of babies arrive on their estimated due date. A large Scandinavian study evaluating more than 14,000 spontaneous singleton deliveries tested both the standard 280-day assumption and a 282-day assumption to see which better predicted actual delivery, reflecting the reality that average gestation may be a couple of days longer than the textbook figure.¹⁰ Clinicians generally stick with 280 days for simplicity, but the point is that even the “average” is debatable.

What Affects How Long Your Pregnancy Lasts

Several factors nudge gestation length in one direction or the other. The timing of implantation is one of the most striking. After fertilization, the embryo takes several days to travel down the fallopian tube and embed in the uterine lining. Most implantation happens on days eight, nine, or ten after ovulation, but embryos that implant later have a higher risk of pregnancy loss. Among embryos that implanted by day nine, about 13 percent ended in early loss; that figure climbed to roughly a quarter on day 10 and above half on day 11.¹¹ Later implantation also tends to extend the total duration of gestation slightly, so the day the embryo settles in can shift the eventual delivery date.

Maternal age plays a role too. Research has found that older mothers tend to carry slightly longer, with the length of pregnancy increasing by a small but measurable amount for each additional year of age.¹² Prior pregnancies, body mass index, and even genetics all contribute to the 37-day spread described earlier. None of these factors is large enough to dramatically change a due date on its own, but they help explain why your pregnancy may run shorter or longer than a friend’s even when both are perfectly healthy.

When Ovulation Is Not on Day 14

A persistent misconception is that ovulation reliably occurs at the midpoint of the cycle. In reality, a prospective study of women with known cycle lengths found that the fertile window occurred across a wide swath of the cycle, spanning day 6 through day 21 at a minimum. On any given day in that range, at least one in ten women had a chance of being fertile. And a small fraction of women whose cycles had not yet resumed were still potentially fertile in the fifth week.¹³ When ovulation is this variable, any LMP-based formula that assumes day 14 can be off by a week or more before the pregnancy even starts.

Separate research confirmed that conception occurs only during a roughly six-day window ending on ovulation day itself. The probability of conceiving from a single act of intercourse ranged from about 10 percent five days before ovulation to about 33 percent on the day of ovulation.¹⁴ Even when you know you conceived from a specific encounter, sperm can survive several days inside the body, so the fertilization event itself could have happened days after intercourse. This layered uncertainty is why doctors consider the LMP a rough guide rather than a precise anchor.

The Tape Measure Method

In settings where ultrasound is not available, or when a woman presents for care later in pregnancy, clinicians sometimes rely on symphysis-fundal height (SFH), a tape-measure reading from the pubic bone to the top of the uterus. After about 20 weeks, the measurement in centimeters roughly tracks the number of weeks of gestation. While it is a useful screening tool, systematic reviews have found that ultrasound-based measurements estimate gestational age more accurately than SFH, particularly after 20 weeks.⁴

That said, in resource-limited settings, the tape measure outperforms the LMP alone. A study in urban Pakistan found SFH was more accurate than the reported last menstrual period for estimating gestational age, though neither approached the precision of ultrasound.¹⁵ The clinical bottom line is that SFH remains valuable for tracking growth and flagging discrepancies but should not be the primary method for setting a due date when ultrasound is an option.

Home Pregnancy Tests With Conception Indicators

Some digital home pregnancy tests now display a “weeks since conception” estimate alongside the positive result, using the concentration of the pregnancy hormone hCG in urine as a proxy for how far along you are. One study found that the agreement between this test’s results and the time since ovulation was about 93 percent overall, and when adjustments were made for known measurement variability in the comparison ultrasound, agreement reached 99 percent.¹⁶

These indicators are designed for very early pregnancy, typically distinguishing between “one to two weeks,” “two to three weeks,” and “three-plus weeks” since conception. They can give you a rough sense of timing before your first prenatal visit, but they are not a substitute for a clinical dating scan. The broad categories they offer are not fine-grained enough to pin down a due date, and hCG levels vary considerably from person to person. Think of them as a ballpark confirmation rather than a calendar tool.

How Ovulation, Implantation, and LMP Compare Head to Head

When researchers have had the rare luxury of knowing the exact day of ovulation (through daily hormone tracking) and the exact day of implantation (through hCG detection), they can compare all the dating methods directly. One such study found that the median gestational length was 279 days from LMP, 278 days from first-trimester CRL, and about 276.5 days from either ovulation or implantation.¹⁷ The medians were close, but the spread around each median told a different story. The distributions were widest when gestational age was assigned from CRL or LMP and narrowest when assigned from implantation or ovulation day. In plain terms, if you could measure from the biological starting gun (ovulation or implantation), you would get a tighter prediction of delivery. LMP and even ultrasound add noise because they are only proxies for those true biological events.

This finding reinforces a central point: no method of dating pregnancy removes the inherent unpredictability of when labor will start. The tools we have are good enough for clinical decision-making, but they will never replace the body’s own timetable.

Blood Biomarkers and the Search for a Better Prediction

Researchers have been hunting for a blood test that could predict when labor will begin, particularly to identify pregnancies at risk of delivering too early. A systematic review identified 278 unique biomarkers studied across 77 research articles for their ability to predict spontaneous preterm birth. The most commonly investigated markers included those already measured during routine screening for chromosomal conditions, along with inflammatory proteins. No single biomarker emerged as a clear standalone predictor, though models combining multiple proteins performed better, with the top-performing combination reaching an area-under-the-curve score of 0.89, which is considered quite good for a screening test.¹⁸

For now, these tools remain in the research phase. None has entered routine clinical use for predicting either preterm birth or full-term delivery timing. But the direction of the science suggests that future prenatal care may eventually incorporate a blood draw that narrows the delivery window beyond what ultrasound and LMP alone can offer. Whether that happens within the next decade depends on whether multi-marker panels can be validated in large, diverse populations and made cost-effective enough for widespread screening.

1
PubMed. Naegele’s rule and the length of pregnancy – A review
2
Research Square. Generative AI systems conflate two distinct pregnancy-dating algorithms
3
PubMed. Menstrual cycle length and the calculation of gestational age
4
PubMed Central. Second and third trimester estimation of gestational age using ultrasound or maternal symphysis-fundal height measurements: A systematic review
5
PubMed. A comparison of first trimester measurements for prediction of delivery date
6
PubMed. Accuracy of ultrasound dating formulae in the late second-trimester in pregnancies conceived with in-vitro fertilization
7
PubMed Central. Committee Opinion No 700: Methods for Estimating the Due Date
8
PubMed Central. Length of human pregnancy and contributors to its natural variation
9
Human Reproduction. Length of human pregnancy and contributors to its natural variation
10
PubMed. Evaluation of ultrasound-estimated date of delivery in 17,450 spontaneous singleton births: do we need to modify Naegele’s rule?
11
New England Journal of Medicine. Time of implantation of the conceptus and loss of pregnancy
12
PubMed Central. Factors affecting the duration of gestation among women taking prenatal care at Gondar referral hospital, Ethiopia
13
PubMed Central. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study
14
New England Journal of Medicine. Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby.
15
PubMed Central. Dating gestational age by last menstrual period, symphysis-fundal height, and ultrasound in urban Pakistan
16
Elsevier / Fertility and Sterility. Accuracy of new clearblue digital pregnancy test with conception indicator
17
PubMed Central. Gestational length assignment based on last menstrual period, first trimester crown-rump length, ovulation, and implantation timing
18
PLOS ONE. Is there a maternal blood biomarker that can predict spontaneous preterm birth prior to labour onset? A systematic review