About 4 to 5 percent of babies are born on their exact due date. That is roughly 1 in 22. A due date is the center of a probability window, not an appointment: around 60 percent of babies arrive within a week of it, and close to 90 percent within two weeks.
So if you just ran your dates through our pregnancy due date calculator and got a specific Tuesday, treat that Tuesday as the peak of a curve, not a promise. It is the best single guess the math can produce. It is also wrong about 95 times out of 100.
People keep asking what percentage of babies are born on their due date because the answer feels like it should be higher. It is not, and the reasons are worth two minutes of your time.
Why only 4 to 5 percent land on the exact day
The standard estimated date of delivery is your last period plus 280 days. That formula, Naegele’s rule, dates to the 1800s and assumes every cycle runs 28 days with ovulation on day 14. Real cycles disagree. Ovulation can shift by a week without any outward sign, which quietly moves the true start of the pregnancy before anyone has measured anything.
Biology then adds its own spread on top. In one well-known study that tracked ovulation directly, healthy pregnancies still varied in length by about five weeks. Babies do not read calendars.
We covered the formula side in our guide to how a due date is calculated. This article is about the other half: how often the estimate turns out to be right.
When babies actually arrive, week by week
A full term pregnancy spans 37 to 42 weeks, and doctors now slice that range into named bands because outcomes differ across it. Here is roughly how births spread out in current US data.
| Timing | Label | Approximate share of births |
|---|---|---|
| Before 37 weeks | Preterm | About 10% |
| 37 to 38 weeks | Early term | About 26 to 29% |
| 39 to 40 weeks | Full term | About 55 to 57% |
| 41 weeks | Late term | About 5 to 6% |
| 42 weeks or later | Post term | Under 1% |
Notice the bulge at 39 to 40 weeks. Part of it is biology, since spontaneous labor genuinely peaks near 40 weeks. Part of it is scheduling, because inductions and planned cesareans cluster at 39 weeks. Going past 42 weeks was once fairly common; today most providers offer induction by 41 weeks, so post term births have become rare.
Add it all up and your chances of going into labor before your due date sit slightly better than even.
How accurate are due dates in the first place
It depends on how the date was set. A first-trimester ultrasound, which measures the embryo from crown to rump, pins gestational age to within about 5 to 7 days. A date built only from your last period can miss by two weeks or more, since it leans entirely on that 28-day-cycle assumption.
That gap is why most due date accuracy statistics favor early scans, and why standard practice is to redate the pregnancy when a first-trimester scan disagrees with the period date by more than about a week. Later scans get progressively fuzzier. By the third trimester an ultrasound can be off by three weeks either way, so nobody redates from those without good reason. This matters everywhere, but especially in places where early scans are not routine and dates often come from the period alone.
If a redate has left you unsure where you stand, our explainer on how many weeks pregnant you are walks through the counting.
A worked example: turning the date into odds
Say the calculator gives you November 14. Here is the honest way to read it.
Step 1, the exact day. Take the middle of the 4 to 5 percent range and call it 4.5 percent. That is about 1 chance in 22, worse odds than calling four coin flips in a row. Honestly, nobody should plan anything around a 1 in 22 chance.
Step 2, the one-week window. Count seven days on each side, November 7 through November 21. About 60 percent of babies arrive inside that stretch. Babies born within a week of the due date are the norm, in other words, even though the day itself is a long shot.
Step 3, the two-week window. October 31 through November 28 catches roughly 90 percent of births. Nine times out of ten, the baby shows up somewhere in that band.
Step 4, the leftover. That still leaves about a 10 percent chance of arriving outside even the wide window, and most of that sits on the early side. This is why hospital bags get packed at 36 weeks. (Pack it anyway. Nobody has ever regretted an early bag.)
Do first babies come early or late
Family folklore says first babies come late, and for once the folklore matches the data. First pregnancies run a few days longer on average. When labor starts on its own, more than half of first-time moms are still pregnant past the due date, and reaching 41 weeks is common enough that providers plan for it.
Second and later babies tend to arrive a little earlier. Your own history counts too, because the length of one pregnancy is a decent predictor of the next. If your first came at 40 weeks and 4 days, do not expect number two at 38 weeks.
Frequently asked questions
What percentage of babies are born on their exact due date?
About 4 to 5 percent of babies are born on their exact due date, roughly 1 in 22. The estimated date is the statistical midpoint of a wide window, so a hit on the day itself is uncommon. Around 60 percent of babies arrive within a week of the date, and close to 90 percent within two weeks.
How accurate are due dates from an ultrasound vs the last period?
A first-trimester ultrasound is accurate to about 5 to 7 days, which makes it the most reliable dating method. Dating from the last menstrual period assumes a 28-day cycle with ovulation on day 14, so it can miss by two weeks or more when cycles are irregular. Later ultrasounds are progressively less precise.
What are the chances of going into labor before your due date?
Slightly more than half of babies are born before the due date. In US birth data, roughly 10 percent arrive before 37 weeks and about a quarter arrive during weeks 37 and 38, with many more coming in week 39. First pregnancies are the exception, since they tend to run a few days longer.
Do first-time moms usually deliver early or late?
First-time moms usually deliver late, by a few days on average. When labor starts on its own, more than half of first pregnancies go past the due date, and going to 41 weeks is common. Later pregnancies tend to run slightly shorter, and your own previous timing is a useful guide.
What percentage of babies are born within a week of the due date?
About 60 percent of babies are born within a week of the due date, meaning the stretch from 39 weeks 0 days to 41 weeks 0 days. Widen the window to two weeks on either side and the figure climbs to roughly 90 percent. The exact day itself catches only 4 to 5 percent.
Why did my due date change after my ultrasound?
Due dates change after an ultrasound because early measurements of the embryo date a pregnancy more precisely than a period date can. If the scan differs from your period-based date by more than about 5 to 7 days in the first trimester, providers redate to the ultrasound. The new date is the better one.
Your due date is a bell curve with a label on the middle. Get your own date from the free pregnancy due date calculator, then mark the whole fortnight around it instead of circling one square. The baby has the final say anyway.