Due date calculator: your delivery date, current week and scan schedule

Work out your due date

Enter the first day of your last period and your usual cycle length. You get your estimated due date, the week and day you are in today, and the dates your main scans fall due.

How your due date is calculated

Your estimated due date is the first day of your last period plus 280 days, which is 40 weeks. This is Naegele's rule, and it is the method hospitals in India and elsewhere use as the starting point.

The count begins at your last period, not at conception. On the day you actually conceive, the chart already reads two weeks pregnant. Doctors count this way for a practical reason: the date a period started is known, and the moment of conception is not.

Actual gestation is therefore closer to 38 weeks than 40. In the study that followed naturally conceived pregnancies from the day of ovulation, the median time from ovulation to birth was 268 days, or 38 weeks and 2 days. Your file, your scans and your doctor will still speak in the 40-week count, so this tool does too.

The 280 days assume a 28-day cycle. Yours may not be 28 days, which is why this calculator asks for your cycle length and adjusts the date. Most free calculators do not ask.

Why your cycle length changes the date

Cycles anywhere from 21 to 35 days are normal. The part that matters for dating is when you ovulate.

The egg is released about 14 days before the next period, whatever the length of the cycle. On a 32-day cycle that is around day 18, not day 14. So a cycle four days longer moves the true due date four days later.

On a long or irregular cycle, a date taken from the period alone can be a week out. Those are the pregnancies most likely to be told the baby is measuring small when nothing is wrong.

A first-trimester scan is more accurate than any calculation

Up to and including 13 weeks 6 days, measuring the baby's crown-rump length dates the pregnancy to within 5 to 7 days. At that stage every baby grows at close to the same rate, which is what makes the measurement reliable.

When the scan and the period date disagree, the scan usually decides, but the margin depends on how early the scan is done. Before 9 weeks, the date is changed if the two differ by more than 5 days. From 9 weeks to 15 weeks 6 days the margin is 7 days, then 10 days to 21 weeks 6 days, 14 days to 27 weeks 6 days, and 21 days after that.

Once the date is changed, it stays changed for the rest of the pregnancy. If the date in your hospital file differs from the one here, this is almost always the reason, and your file is the one to use.

A later scan cannot re-date a pregnancy. By the third trimester healthy babies vary so much in size that age estimated from measurements can be two to three weeks out. A late scan checks growth, not dates.

What this calculator cannot tell you

It cannot tell you the baby's sex. In India, determining or disclosing the sex of a fetus is a criminal offence under the Pre-Conception and Pre-Natal Diagnostic Techniques Act, 1994. No calculator, scan or clinic may do it.

It cannot predict the baby's weight, whether you will deliver vaginally or by caesarean, or whether the pregnancy will run to term.

It does not replace a dating scan. It gives you the same starting estimate your doctor begins with, and nothing beyond that.

The due date is a midpoint, not a deadline

Birth between 37 and 42 weeks is normal, and obstetric guidance divides that span rather than treating it as one block: 37 0/7 to 38 6/7 weeks is early term, 39 0/7 to 40 6/7 weeks is full term, 41 0/7 to 41 6/7 weeks is late term, and 42 weeks and beyond is post-term.

Normal pregnancies vary in length far more than parents are usually told. Among naturally conceived pregnancies dated from ovulation, with complications and preterm births excluded, the spread between the shortest and the longest was 37 days.

Very few babies arrive on the estimated date itself. Treat it as the middle of a five-week window, not as an appointment.

First babies tend to come later. A due date passing means nothing is wrong, though after 41 weeks your doctor will monitor the baby's movements and the fluid around it, and will usually discuss induction between 41 and 42 weeks.

One practical suggestion for the family. Give relatives the last week of the month rather than the exact date. Announce a precise date and the daily phone calls begin on it.

What this date decides

The whole antenatal calendar is scheduled against this one number, which is why it is worth getting right early.

The NT scan can only be done between 11 and 13 weeks 6 days; outside that window the measurement cannot be used. The anomaly scan, the detailed check of the baby's organs, falls between 18 and 22 weeks. The glucose tolerance test comes at 24 to 28 weeks, and the tetanus injections follow the same arithmetic.

The World Health Organization recommends at least one ultrasound before 24 weeks for every pregnancy, both to date it and to check the baby's development.

Record the date in your phone rather than your memory, and know your current week at every visit. At government facilities, the Pradhan Mantri Surakshit Matritva Abhiyan clinic on the 9th of every month offers a free antenatal check-up if you want a second opinion on your dates.

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