How Accurate Is Your Due Date?
The Truth About Due Dates
When your healthcare provider announces your due date, it can feel like a firm deadline. In reality, a due date is an estimate — a statistical midpoint of a normal range, not a precise prediction. Studies consistently show that only about 4 to 5 percent of babies are born on their exact estimated due date. Most births occur within a two-week window on either side of that date.
Understanding how due dates are calculated — and where the uncertainty comes from — can help you approach the end of pregnancy with realistic expectations.
How Due Dates Are Calculated: Naegele’s Rule
The most widely used method for calculating a due date is Naegele’s Rule, developed by German obstetrician Franz Karl Naegele in the 19th century. The formula is straightforward: take the first day of your last menstrual period (LMP), add one year, subtract three months, and add seven days. The result is your estimated due date — 280 days, or 40 weeks, from your LMP.
This formula assumes a 28-day menstrual cycle with ovulation occurring on day 14. In practice, cycle lengths vary considerably. Women with longer cycles (say, 35 days) will typically ovulate later than day 14, meaning conception happens later than the formula assumes and the due date calculated from LMP will be a few days earlier than it should be.
The Limitations of LMP-Based Dating
Naegele’s Rule has several built-in sources of error:
- Cycle length variation. The formula assumes a 28-day cycle, but cycles of 21 to 35 days are all within the normal range.
- Irregular cycles. Women with polycystic ovary syndrome (PCOS), recent hormonal contraceptive use, or stress-related cycle disruption may have highly variable cycle lengths.
- Recall accuracy. Many women are uncertain about the exact date of their last period, especially if they were not tracking their cycle.
For women who know their exact date of ovulation or conception — through ovulation tracking, fertility treatments, or IVF — a conception-based calculation is more precise.
Ultrasound Dating: The Gold Standard
Early ultrasound, performed between 8 and 14 weeks of pregnancy, is considered the most accurate method of gestational dating. During this window, fetal size is relatively uniform across pregnancies — babies grow at predictable rates in early development regardless of genetics. The sonographer measures crown-rump length (CRL) and uses standardized charts to estimate gestational age.
According to ACOG Committee Opinion No. 700, a first-trimester ultrasound is accurate to within 5 to 7 days. If the ultrasound date differs from the LMP-based date by more than 7 days in the first trimester, the ultrasound date should be used instead.
After 14 weeks, accuracy decreases because fetal size becomes more influenced by individual genetics and growth patterns.
Why Most Babies Don’t Arrive “On Time”
Normal human pregnancy length is not a single fixed number — it is a distribution. Research published in the British Medical Journal found that even when women know their exact date of ovulation, the time from ovulation to birth varies by as much as five weeks. The 40-week mark is simply the peak of the bell curve.
First-time mothers tend to deliver slightly later than their due date on average. Subsequent pregnancies often arrive a few days earlier. Genetic and ethnic factors also play a role.
What Your Due Date Is Actually Good For
Even with its limitations, the estimated due date serves important clinical purposes:
- It guides the timing of prenatal screenings and tests.
- It helps clinicians identify pregnancies that may be progressing too slowly or too quickly.
- It establishes a reference for determining when a pregnancy is “post-term” (typically beyond 42 weeks), which carries increased risks and may prompt induction.
Think of your due date as the center of a target, not the bullseye you must hit. Most healthy babies arrive within the two weeks before or two weeks after — all of which falls within the normal range of a full-term pregnancy.