Pregnancy due date calculator

How the due date is calculated

The standard method (Naegele's Rule) adds 280 days (40 weeks) to the first day of your last menstrual period. Doctors and midwives use the same rule for a first estimate, before a dating scan. If your cycle is not 28 days long, enter your usual cycle length: the calculator moves the date by the difference (for a 32-day cycle, four days later).

The 40-week count starts from your LMP - even though conception typically happens around week 2. So your "gestational age" as measured medically is always about 2 weeks ahead of your embryonic age.

Trimesters

• First trimester: Weeks 1–12 (LMP to ~84 days)
• Second trimester: Weeks 13–26 (~85 to ~182 days)
• Third trimester: Weeks 27–40 (~183 days to due date)

How accurate is this date?

Only about 4% of babies are born on their exact due date. The estimated due date (EDD) is the midpoint of a normal delivery window that typically spans from 38 to 42 weeks. A dating scan in the first trimester gives a more accurate date, particularly when your cycle length differs from the standard 28-day assumption or you are unsure of your last period.

Sources

Naegele's rule and ultrasound dating: American College of Obstetricians and Gynecologists, Committee Opinion 700, Methods for Estimating the Due Date. For a second opinion on the date, see the NHS due date calculator and Tommy's due date calculator (Tommy's notes that only around 4 in 100 babies are born on their due date). This tool is not medical advice: ask your midwife or doctor about your pregnancy.

Track the countdown with Left

Add your due date as a countdown in Left and pin it to your Home Screen or Lock Screen. Every morning you'll see exactly how many days remain - and after the birth, switch to Left's "Since" mode to track how many days old your baby is.

To keep the due date in sight, add a countdown widget to your Home Screen with Left.