Due Date and Gestational Age Calculator

Key weeks and the dates they fall on

A due date is calculated from the first day of your last menstrual period, which is the starting point obstetricians use worldwide. What sets this calculator apart from most is that it adjusts for your actual cycle length. If your cycle is not 28 days, ovulation happens earlier or later than the standard formula assumes, and the due date shifts by the same number of days.

Naegele’s rule is the standard method, in use for over two hundred years: take the first day of the last menstrual period and add 280 days, or 40 weeks. An equivalent shortcut is to subtract three months, add seven days, and add a year.

The reason dating starts from the period rather than conception is simple: almost everyone knows when their last period began, while the actual date of conception is usually unknown. One consequence is that gestational age counted this way is about two weeks greater than the embryo’s actual age. That is normal and every clinician uses the same convention.

Adjusting for cycle length. The 280-day figure assumes a 28-day cycle with ovulation on day 14. With a 32-day cycle, ovulation occurs roughly four days later, so the due date should move four days later too. This calculator makes that adjustment automatically once you enter your real cycle length.

How gestational age is counted. In weeks and days from the first day of the last period to today. "12 weeks and 3 days" means 87 days have passed. This is the notation used in maternity records and at every appointment.

Trimesters. The first runs from week 1 to 13, the second from week 14 to 27, and the third from week 28 until birth.

What the number cannot tell you. Only about 4% of babies arrive on their due date. Birth anywhere from week 37 to 42 is within the normal range, so the due date is better understood as the centre of a window than as an appointment.

A first-trimester ultrasound also dates a pregnancy more accurately than this calculation, particularly where cycles are irregular or the last period is uncertain. If the ultrasound date differs from the one here, the date your doctor gives you is the one to use.

This is not a medical assessment. It applies a standard formula for general reference. Nothing here replaces antenatal care and the appointments your doctor schedules.

If you are pregnant in Thailand, note that antenatal care is covered under social security for insured people, and public hospitals provide it under the universal coverage scheme. Foreign residents should check what their insurance covers early, since maternity is commonly excluded or subject to a waiting period.