How accurate is the due date?
About 4% of babies arrive on their due date. So what's the date actually for?
Pregnancy · Explainer
TL;DR
About 4% of babies are born on their exact due date. Roughly 90% arrive between weeks 38 and 42. The "due date" is a midpoint, not a deadline. First-trimester ultrasound is the most accurate dating method, often more accurate than the last-period date. Full term begins at 37 weeks, late term at 41, post-term at 42.
The way people talk about the due date makes it sound precise. Friends ask when it is. Showers get planned around it. Leave dates get pegged to it. But it's a guess built from one data point (your last period) and a 280-day rule that assumes everyone has identical cycles. Most don't.
How the due date is calculated
The standard formula is Naegele's rule, named after a 19th-century German obstetrician:
Due date = first day of last menstrual period + 280 days
That's 40 weeks. The math assumes a 28-day cycle with ovulation on day 14. If your cycle is 30 days, your real due date is roughly 2 days later than the formula gives. A 35-day cycle? About 7 days later.
This is why the "due date" varies depending on who's calculating it. Your OB might use Naegele's rule. A midwife or app might use Mittendorf's rule, which adds 15 days for first-time pregnancies (it isn't widely used clinically anymore, but it's out there).
Why the date is so imprecise
Three reasons the LMP-based math is rough:
1. Cycle variation
"Average" cycle is 28 days. Most people's cycles run 24–35. Some are very irregular. The math doesn't know yours.
2. Ovulation timing
Even within one person's cycle, ovulation drifts. You might ovulate on day 12 one cycle and day 17 the next. The math assumes day 14 every time.
3. Implantation timing
After fertilization, the embryo takes 6–10 days to implant. There's variability there too.
Stack those together and the LMP-derived due date can be off by 1–2 weeks from "true" gestational age. Which is why, in clinical practice, an early ultrasound is often used to "redate" the pregnancy.
Try the calculator with your cycle length
Adjust for non-28-day cycles, or use ultrasound or IVF transfer date for the most accurate estimate.
Calculate due dateThe most accurate dating method: first-trimester ultrasound
Between weeks 8 and 13, an ultrasound measures the crown-rump length of the embryo. This length tracks gestational age tightly: accurate to within 5–7 days.
ACOG's clinical guidance is straightforward: if the LMP date and the first-trimester ultrasound differ by 7+ days before week 13, the ultrasound date wins. After that, dating doesn't change unless there's a major discrepancy.
Earlier ultrasound = more accurate dating. By the third trimester, all babies are roughly the same size regardless of gestational age, and ultrasound dating becomes much less reliable.
The most accurate method, period: IVF
If you conceived through IVF, the date of fertilization is known to the day. Day-5 transfer plus 261 days. Day-3 transfer plus 263. IVF dating is so precise that it's rarely changed by ultrasound.
The bell curve of birth dates
Here's what actual US birth statistics look like for full-term singletons:
| Week | % of births |
|---|---|
| Before week 37 (preterm) | ~10% |
| Week 37 | ~6% |
| Week 38 | ~14% |
| Week 39 (most common) | ~28% |
| Week 40 (the due date) | ~24% |
| Week 41 | ~12% |
| Week 42+ (post-term) | ~5% |
The peak is week 39, one week before the due date. Partly because many late babies are induced before week 42, and partly because 39-week birth is just genuinely common.
What "late" actually means
Week 37–38: early term
Officially full-term but on the early end. No interventions if labor starts on its own.
Week 39–40: full term (peak)
The bell-curve peak. Most babies arrive here.
Week 41: late term
Still considered normal. Doctors increase monitoring with non-stress tests and amniotic fluid checks. Some start discussing induction.
Week 42+: post-term
Induction is generally recommended at 41+5 to 42 weeks. The risks of going further include placental aging, low amniotic fluid, and stillbirth (which becomes statistically meaningful past 42 weeks).
Why the due date matters less than you think
Functionally, the date is useful for two things: clinical decisions (when to schedule tests, when to consider induction) and logistical planning (leave start, hospital pre-registration). It does not predict your specific birth date.
Use it as a window, not a deadline. Be done with anything time-critical (work projects, prep, hospital bag) by week 36. Plan for the baby to arrive anywhere from week 37 to week 41. And brace yourself emotionally for the possibility of going past, because it's common and it's hard.
The late-pregnancy waiting game
Going past your due date is its own thing. About 40% of first-time pregnancies do. The wait is harder than the wait before the due date because everyone's now actively asking. Phone calls. Texts. The "any updates??" message from your aunt.
Things that genuinely help:
- Tell people the baby is due "around mid-month," not on the actual date
- Stop the daily check-ins past week 40. Keep doing your normal life as much as possible
- If your provider offers it, a membrane sweep at 39+ weeks can sometimes nudge labor along
- Walk daily. It won't induce labor, but it makes the waiting bearable
The end of pregnancy is somewhere between weeks 37 and 42. Plan for the range, not the date.