Pregnancy Calculator

How the pregnancy calculator works

Choose how your pregnancy is dated. With the last menstrual period, the calculator applies Naegele's rule: due date = first day of the last period + 280 days (40 weeks), shifted by the difference between your cycle length and 28 days, because a longer cycle means later ovulation. With an early ultrasound, the age measured at the scan is carried forward: due date = scan date + (280 - age at the scan in days). With a conception date, gestational age = days since conception + 14 and due date = conception + 266 days. For IVF, a day 5 transfer counts as 19 days of gestational age and a day 3 transfer as 17, with the due date 261 or 263 days after the transfer.

Dating from an early ultrasound, taken before about 14 weeks, is usually the most accurate, and doctors normally keep it when it differs from the last-period estimate by more than about a week, so use it whenever you have a reliable measurement. The result shows the gestational age in completed weeks and days on the date you choose, the trimester (first up to 13 weeks and 6 days, second up to 27 weeks and 6 days, then the third), the pregnancy month, the days left until the due date and an estimated conception date. Only about 4 to 5 percent of babies are born exactly on the due date, and a birth between 37 and 42 weeks is considered normal.

The milestone table lists typical prenatal-care windows, counted from the estimated first day of the last period: first-trimester screening between 11 weeks and 13 weeks 6 days, the anatomy scan between 20 and 24 weeks 6 days, the glucose test between 24 and 28 weeks 6 days, the start of the third trimester at 28 weeks, and early term, full term, late term and post-term from 37, 39, 41 and 42 weeks. The status column compares each window with the gestational age on the calculation date. These windows are general references only: protocols vary between countries and clinics, so follow the schedule given by your care team.

Example: a last period starting on March 1, 2026 and a 28-day cycle give a due date of December 6, 2026 (March 1 + 280 days). On June 10, 2026 the gestational age is 101 days, which is 14 weeks and 3 days, already in the second trimester. With a 32-day cycle the due date would move four days later, to December 10, 2026. The calculation assumes a single pregnancy and fairly regular ovulation: twins, irregular cycles and clinical findings can change the dating. The result is an estimate and never replaces your doctor's assessment.