How Accurate Is a Due Date? What the Evidence Shows

Dating accuracy: how far along you are

Dating answers “how many weeks pregnant am I?” IVF dates are known to the day. A first-trimester ultrasound measuring crown-rump length is accurate to within about 5 to 7 days, and is the most accurate method when IVF dates are not available.¹ A period-based date can be just as good if the period date is certain and cycles are regular, and less reliable when they are not.

Birth prediction: when the baby arrives

Even a perfectly dated pregnancy does not have a fixed length. In an Australian study of 18,708 single births after spontaneous labor, about 5% happened on the estimated due date and about two-thirds within seven days either side.² That study looked only at labor that started on its own, in one population; it doesn’t give personal odds.

A smaller US study dated 125 pregnancies from measured ovulation. Even after excluding preterm births and complicated pregnancies, the time from ovulation to birth varied by 37 days, with a median of 268 days, close to the 266 days the due date assumes.³

When spontaneous labor started, by completed week of pregnancy
Completed weeks	Share of spontaneous labor onsets
Before 37 weeksPreterm	7.7%
37 weeksEarly term	5.1%
38 weeksEarly term	12.1%
39 weeksFull term	25.4%
40 weeksFull term	32.5%
41 weeksLate term	16.2%
42 weeks or morePostterm	0.9%

When spontaneous labor started, by completed week of pregnancy

Before 37 weeksPreterm

7.7%

37 weeksEarly term

5.1%

38 weeksEarly term

12.1%

39 weeksFull term

25.4%

40 weeksFull term

32.5%

41 weeksLate term

16.2%

42 weeks or morePostterm

0.9%

Reading the chart

The chart shows when labor started on its own in NHS hospitals in England in 2019–20, by completed week. The 40th week is the most common, but most labor still starts in another week. It leaves out planned cesareans and inductions, so it is not the chance that any individual gives birth in a given week.⁴

So why have a due date at all?

Because the date anchors care. It times screening tests and scans, tells clinicians whether a baby born early is preterm, and decides when a pregnancy has gone on long enough to discuss induction.¹ Its job is to be consistent, set once and early, rather than to predict a birthday. Thinking in term windows helps: full term runs from 39 weeks 0 days to 40 weeks 6 days.⁵

What a calculator can’t do

No calculator, including this one, can predict your baby’s birthday from a date. Claims of “more accurate” predictions from the same inputs aren’t supported by the evidence. What a good calculator can do is get the arithmetic exactly right and be honest about the uncertainty.

How accurate is your due date?