How Your Due Date Is Calculated

A pregnancy is conventionally dated as 40 weeks, or 280 days, from the first day of your last menstrual period (LMP). This is the standard clinicians use because the LMP is usually easier to pinpoint than the moment of conception. It does mean the count technically starts about two weeks before you actually conceived, which is simply a shared convention rather than a mistake.

The classic shortcut is Naegele's rule: take the first day of your LMP, subtract three months, then add seven days and one year. An LMP of March 10 gives a due date of December 17 the following year. A due date calculator automates this and adjusts for months of different lengths.

If you know your conception or ovulation date, the estimate uses 266 days (38 weeks) instead of 280, since it skips the two-week head start built into LMP dating. This can be more accurate when your cycle is irregular or much longer or shorter than the assumed 28 days.

IVF gives the most precise starting point because the timing is known. For a day-5 blastocyst transfer, the due date is about 261 days from transfer; for a day-3 embryo, about 263 days. A calculator that offers an IVF option asks for the transfer date and embryo age so it can apply the right offset.

Pick the method that matches the date you know most reliably.

Only about one in twenty babies arrives on the exact due date. A full-term birth is anything from 37 to 42 weeks, so the due date marks the center of a wide, normal window rather than a deadline. Early ultrasound measurements, especially in the first trimester, often adjust the date and are generally considered more accurate than LMP dating.

Trimester dates shown by a calculator are approximate boundaries used to track development and appointments. This tool is for planning and curiosity, not medical care. Your due date, screening schedule, and any concerns should always be confirmed with your doctor or midwife.