Why count from the last period rather than conception?

Because the date of your last period is known, while ovulation and fertilisation almost never are. By convention a pregnancy therefore “starts” two weeks before conception, which is why it lasts 40 weeks.

My cycle is irregular. Is the calculation reliable?

Less so. If your cycles vary a lot, an early ultrasound dates the pregnancy more reliably than any calculation. Enter your average cycle length for a first estimate.

Gestational weeks or weeks since conception?

This tool counts gestational weeks (since your last period), the measure doctors use. Weeks since conception start two weeks later.

What does “full term” mean?

Between 37 and 42 weeks. Before 37 weeks a birth is premature; after 41 weeks follow-up becomes closer and induction may be offered.

Free

Due date calculator

Enter the first day of your last period (or your conception date, or your embryo transfer date) to see your estimated due date and where you are today.

Free, no sign-up, nothing stored

First day of your last periodCycle length (days)

Enter a date to see your due date.

Due date calculator

How it works

The due date follows Naegele's rule: 280 days (40 weeks) after the first day of your last period, adjusted if your cycle is not 28 days. From a conception date, 266 days are added. After IVF, the count starts from the transfer date and the age of the embryo.
Only about 4 to 5% of babies arrive on the exact date: most are born between 37 and 42 weeks. The first ultrasound (around 11 to 13 weeks) gives a more accurate dating, which your doctor will use if it differs by more than a few days.
In Thailand, the Ministry of Public Health recommends at least 5 antenatal visits (the WHO now advises 8 contacts), the first before 12 weeks. Antenatal care is covered in public hospitals for people under the universal coverage scheme.
This tool gives an estimate: only a health professional can confirm your due date and follow your pregnancy.

These tools give general information. They do not replace the advice of a doctor or a qualified professional.

Go furtherPregnancy & postpartum

Frequently asked questions

Why count from the last period rather than conception?: Because the date of your last period is known, while ovulation and fertilisation almost never are. By convention a pregnancy therefore “starts” two weeks before conception, which is why it lasts 40 weeks.
My cycle is irregular. Is the calculation reliable?: Less so. If your cycles vary a lot, an early ultrasound dates the pregnancy more reliably than any calculation. Enter your average cycle length for a first estimate.
Gestational weeks or weeks since conception?: This tool counts gestational weeks (since your last period), the measure doctors use. Weeks since conception start two weeks later.
What does “full term” mean?: Between 37 and 42 weeks. Before 37 weeks a birth is premature; after 41 weeks follow-up becomes closer and induction may be offered.

How it works

Frequently asked questions

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