A due date marks the middle of a window
An estimated due date (EDD) is the day a pregnancy reaches 40 weeks. It is calculated from a known date — your last period, conception, IVF transfer or an ultrasound — using standard averages. Those averages are good at describing pregnancies in general, but they cannot predict when labour will start for one particular person. Only a small share of babies, often quoted at around 4 to 5 percent, arrive on their due date. Most are born somewhere in the five weeks from 37 to 42 weeks, so it can help to think of the due date as the centre of a “due window”.
| Timing | Weeks |
|---|---|
| Preterm | Before 37 weeks |
| Early term | 37+0 to 38+6 |
| Full term | 39+0 to 40+6 |
| Late term | 41+0 to 41+6 |
| Post-term | 42+0 and beyond |
How reliable is each dating method?
IVF dates
The most precise. The day of fertilisation and the embryo’s age at transfer are known exactly, so IVF due dates are generally not changed by later scans.
Early ultrasound
A first-trimester scan that measures the crown–rump length is generally considered the most accurate way to date a naturally conceived pregnancy, typically to within about a week. Later scans are less precise for dating, because babies grow at increasingly different rates.
Last menstrual period
Reliable when cycles are regular and the date is certain, but it assumes ovulation on day 14. Long, short or irregular cycles, recent hormonal contraception, breastfeeding and uncertainty about the date all reduce accuracy.
Conception date
Good when ovulation was tracked, but less so when only the date of intercourse is known, because conception may have happened several days later.
Why your due date might change
If your first dating scan measures a gestational age that differs from your LMP-based estimate by more than a set margin — commonly around 5 to 7 days in the first trimester, and wider later — your provider may change your due date to match the scan. This is routine. It usually means ovulation happened earlier or later than the standard assumption, not that the baby is growing unusually. Once a due date has been confirmed by an early scan, it is generally not changed again, even if later scans suggest the baby is measuring bigger or smaller; those differences are interpreted as growth rather than dating.
Things that influence when labour starts
- Whether it is a first pregnancy
- The length of previous pregnancies
- Carrying twins or more, which tend to arrive earlier
- Individual and family factors that are not fully understood
- Medical decisions, such as a planned induction or caesarean birth
None of these can be turned into an exact prediction, which is why your care team plans around the due window rather than a single day.
Making the most of your estimated due date
Use the date to plan — leave from work, support at home, when to pack a bag — but build in flexibility on both sides. Recalculate with the Due Date Calculator in ultrasound mode if your provider gives you a scan-based date, and follow the weeks with the Pregnancy Timeline. Your midwife or doctor is the best person to explain the dating of your own pregnancy.