When will I go into labour?
What birth statistics can tell you about timing, and what they cannot
Last updated
Nobody can predict the day labour starts. Statistics can tell you how likely each day is. About 4 in 100 babies arrive on their due date, about half have arrived by it, and about four in five by 41 weeks. The calculator applies those population patterns to your own due date and personal factors.
See your own odds, day by day
Enter your due date for probabilities fitted to NHS and CDC birth data.
Why a due date is an estimate
Your due date is 40 weeks from the first day of your last period, or adjusted from a dating scan. Healthy pregnancies range from 37 to 42 weeks, so the date marks the middle of a window, not a deadline.
The useful question is not which day, but what the chances are this week. Those odds change every day you stay pregnant, which is why this site shows conditional probability instead of one date.
What the statistics show
4%
born on the due date
~50%
born by the due date
~80%
born by 41 weeks
~100%
born by 42 weeks
Figures for spontaneous labour from NHS Maternity Statistics 2023-24 and CDC natality data for 2023, as fitted by the model on this site.
What shifts your timing
Several factors move the expected date by days. Each one is applied by the calculator and cites its study on the methodology page.
| Factor | Typical effect |
|---|---|
| First pregnancy | About 1 to 2 days later than later pregnancies |
| Baby's sex | Boys about 1 day later than girls |
| BMI | Higher BMI is linked with later delivery, up to 5 days |
| Twins or triplets | Twins about 3 weeks earlier; triplets earlier still |
| Maternal age | Under 25 or over 40: slightly earlier |
| Gap since last birth | Under 12 months: earlier delivery |
How this calculator works
Most due date calculators give one date. This one shows conditional probability: your odds of going into labour given that you are still pregnant. At 41 weeks your daily odds are much higher than at 38 weeks because fewer pregnancies remain.
The model is a Johnson SU distribution fitted to NHS Maternity Statistics 2023-24 and CDC natality data for 2023, covering everything from very preterm to post-term births. The methodology page shows the calibration targets and the studies behind each adjustment.
While you wait
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See your own odds, day by day
Enter your due date for probabilities fitted to NHS and CDC birth data.
Frequently asked questions
When will I go into labour?
No one can predict the exact day. Birth statistics from the NHS and CDC show that about half of babies arrive by the due date and about four in five by 41 weeks. A probability calculator turns your due date into daily and weekly odds of spontaneous labour.
How accurate are labour prediction calculators?
They give population estimates, not personal predictions. Individual prediction has limited accuracy in research (a c-statistic of about 0.56). They are useful for understanding the range of likely dates and planning around it.
Does labour timing differ for a second or third baby?
Yes. First pregnancies tend to run a little later, and second and third pregnancies a little earlier. The calculator adjusts for parity.
What factors affect when I will go into labour?
First-time mothers deliver about 1 to 2 days later on average. Boys arrive about a day later than girls. A higher BMI is linked with later delivery. Twins typically arrive about three weeks early. A short gap since a previous birth is linked with earlier delivery.
What formula does the calculator use?
A Johnson SU distribution fitted to large birth datasets, with conditional probability applied for being still pregnant today. That gives day-by-day and week-by-week odds instead of a single predicted date.