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How we estimate your due date
Our calculator uses Naegele’s rule: it adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method is widely accepted by obstetricians for uncomplicated 28‑day cycles.
Only about 4 % of babies arrive exactly on their estimated due date. Most deliveries occur within a two‑week window before or after. Early ultrasound can fine‑tune the date if your cycle length is different.
Pregnancy Due Date – FAQ
How accurate is the estimated due date?
Only around 4 % of babies are born exactly on their due date. Ultrasound dating in the first trimester is usually accurate within ±5 days.
Does cycle length affect the calculation?
Yes. Naegele’s rule assumes a 28‑day cycle. Longer cycles push the date later; shorter cycles move it earlier.
What if I don’t remember my last period?
A first‑trimester dating ultrasound is the best way to estimate gestational age when the LMP date is uncertain.
Can my due date change later in pregnancy?
It can. If later ultrasounds differ by more than 2 weeks, your provider may update the due date.
Is it normal to give birth after 40 weeks?
Yes. Many pregnancies continue to 41‑42 weeks; doctors monitor closely and may induce after 41 weeks.
What is gestational age?
Gestational age is the length of pregnancy measured from the first day of your last period, not from conception.
When should the first prenatal visit be scheduled?
Ideally, book your first prenatal appointment as soon as you confirm pregnancy – typically around 6‑8 weeks.
Does IVF change how due dates are calculated?
In IVF, due dates are calculated from the fertilisation/transfer date: add 266 days to fertilisation, or 38 weeks to a day‑3 embryo transfer.