A due date calculator does its math accurately. The real uncertainty lies elsewhere. Nobody can predict exactly when labor will start. Research makes this clear. Only about 4% of babies arrive on their exact calculated due date. Even with ultrasound dating, only about 70% arrive within 10 days of the estimate, early or late. So think of a due date as the center of a normal range. It isn’t a predicted day.
Why the Underlying Biology Is the Real Variable
A 2013 study in Human Reproduction tracked ovulation with daily hormone tests. It followed 125 women who conceived naturally. The researchers measured each pregnancy from ovulation to birth. The average was 268 days. That’s close to the 266-day figure clinical formulas assume.
But healthy pregnancies still varied by as much as 37 days. That was true even after researchers excluded preterm births. It was also true even though they measured ovulation precisely. This matters because it shows something important. Dating errors don’t explain all the variation. Healthy pregnancies naturally run for different lengths.
Accuracy by Dating Method
Dating methods carry different amounts of uncertainty. The gaps are large enough to matter.
| Method | Typical accuracy | Main source of error |
|---|---|---|
| LMP (Naegele’s rule) | Within 1–2 weeks | Assumes ovulation on day 14 |
| First-trimester ultrasound | Within about 5–7 days | Small differences in embryo size |
| IVF transfer date | Most precise of the three | Very little |
LMP dating has the widest margin. It relies on an assumption about ovulation timing. Ultrasound narrows the margin by measuring the embryo itself. IVF dating narrows it further. It doesn’t need to estimate ovulation at all.
Why Accuracy Drops as Pregnancy Progresses
Ultrasound dating loses precision as pregnancy moves along. That’s why guidelines set different thresholds for changing a due date. Early on, embryos of the same age look very similar in size. A scan before about 14 weeks can date a pregnancy within roughly 5 to 7 days.
By 16 to 22 weeks, the acceptable margin grows to about 10 days. Babies start to vary more in size at that stage. After 28 weeks, the margin grows to about three weeks. Providers generally consider late scans too imprecise for dating. They use those scans to check growth and wellbeing instead.
This also explains why your due date rarely changes after early pregnancy. A third-trimester scan might show a big or small baby. That reflects size, not a new due date.
Factors That Add Extra Uncertainty
A few situations widen the margin beyond what a standard calculator shows. Irregular cycles weaken the day-14 ovulation assumption. This includes cycles linked to conditions like PCOS. Not knowing your last period date removes the input the formula needs.
Recent hormonal birth control can delay a predictable cycle. Breastfeeding can suppress ovulation for months. Both make a “last period” date unreliable. In these situations, a first-trimester ultrasound removes most of the guesswork. It measures the pregnancy directly. It doesn’t work backward from a period date.
What This Means in Practice
Treat your calculator result as the middle of a normal range. Don’t treat it as a scheduled event. A due date confirmed by first-trimester ultrasound is a solid, well-supported estimate. It’s still an estimate, though. Normal, healthy births happen anywhere from 37 to 42 weeks.
Providers use the date to time screening tests and plan prenatal visits. They don’t use it to predict an exact delivery day. You can approach your own due date the same way.
Frequently Asked Questions
Which due date method is most accurate?
IVF dating is generally the most precise, when it applies. First-trimester ultrasound comes next, within about five to seven days. LMP dating follows, within one to two weeks if cycles are regular.
Why did research find only 70% of babies arrive within 10 days of their due date?
Pregnancy length varies naturally between healthy people. Dating error isn’t the only cause. One study measured ovulation directly. It still found a 37-day natural range in healthy pregnancies.
Does a due date calculator get less accurate the further along I am?
Your original estimate doesn’t change. But ultrasound gets less precise for dating later in pregnancy. That’s why providers use early scans for dating. They use later scans to check growth.