How your due date is worked out
A pregnancy is counted from the first day of your last menstrual period, not from conception - so at the moment of conception you are already “two weeks pregnant”. Adding 40 weeks (280 days) to that date gives the estimated date of delivery, or EDD. If your cycles are longer than 28 days, ovulation happens later and the date moves later by the same number of days; shorter cycles move it earlier.
The most accurate due date comes from an ultrasound in the first trimester, when babies grow at almost exactly the same rate. The crown–rump length measured between 7 and 13+6 weeks dates a pregnancy to within about five days. That is why the dating scan and the NT scan matter even in a healthy pregnancy.
The three trimesters
- First trimester (0–13 weeks): dating scan, NT scan and first-trimester screening, folic acid, and a check of blood pressure, thyroid and haemoglobin.
- Second trimester (14–27 weeks): the level II anomaly scan at 18–22 weeks, fetal echo if advised, and the glucose test for gestational diabetes.
- Third trimester (28–40 weeks): growth and Doppler scans, baby movement awareness and planning for delivery.
Do not wait for a date if you have
- Bleeding or severe pain in the abdomen
- A severe headache, blurred vision or sudden swelling of the face and hands
- Leaking fluid, or the baby moving less than usual
Call +91 90545 74829 - labour and emergency care at Divine Women's Hospital are available 24 × 7.
Frequently asked questions
How is the pregnancy due date calculated?
The usual method (Naegele’s rule) adds 280 days - 40 weeks - to the first day of your last menstrual period, assuming a 28-day cycle. If your cycle is longer or shorter, the date is moved by the difference. If you know the date of conception, 266 days are added to it.
How accurate is a due date calculator?
It is an estimate. Only about 4–5% of babies arrive on the exact due date; most are born within two weeks either side. An early ultrasound between 7 and 13+6 weeks is the most accurate way to date a pregnancy, and your doctor may adjust the date after that scan.
My scan due date is different from the calculator. Which one is right?
If the first-trimester scan differs from your period-based date by more than about 5–7 days, the scan date is generally used, because periods are often irregular or remembered inexactly. Once set from an early scan, the due date is usually not changed by later scans.
When should I book my first pregnancy appointment?
Ideally between 6 and 8 weeks, or as soon as your test is positive if you have a medical condition, a previous complicated pregnancy, pain or bleeding. That visit leaves time to plan the NT scan, which must be done between 11 weeks and 13 weeks 6 days.
Does an IVF pregnancy use a different calculation?
Yes. For IVF the due date is calculated from the embryo transfer date: 266 days minus the embryo’s age at transfer (for example, a day-5 blastocyst transfer adds 261 days). Your fertility team will give you the exact date.