Pregnancy Calculators

About these tools

The Due Date Calculator uses Naegele's rule — the method behind the standard 40-week pregnancy count — adjusted for cycle length. The Ovulation Calculator estimates your fertile window from your cycle. Your healthcare provider will confirm dates, usually with an early ultrasound. The ovulation estimate must never be used as birth control.

How pregnancy dating works

Pregnancy is counted from the first day of the last menstrual period, about two weeks before conception in a typical cycle. That convention is why a full-term pregnancy is described as 40 weeks even though the embryo is roughly 38 weeks old at birth. ACOG recommends confirming the estimated due date with an ultrasound in the first trimester, because it measures the embryo directly; the ultrasound date then takes priority if the two differ by more than a set margin.

Cycle length and ovulation

Both calculators assume ovulation happens about 14 days before the next period. In a 28-day cycle that is day 14; in a 32-day cycle, day 18. Real ovulation varies from cycle to cycle, so calendar dates are a starting point. Ovulation test kits, basal body temperature and cervical mucus changes give more personal information.

Using these tools responsibly

These calculators do not diagnose pregnancy, predict fertility problems or replace prenatal care. They store nothing — the dates you enter stay in your browser. If you think you may be pregnant, contact a healthcare provider to start prenatal care, and seek urgent help for bleeding, severe pain or other worrying symptoms. If you are trying to conceive and it hasn't happened after a year (or six months if you are 35 or older), talk to your provider.

Understanding the trimesters

The first trimester runs to the end of week 13, the second from week 14 to week 27, and the third from week 28 until birth. ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days. The due date calculator shows when each trimester begins for your dates.

Healthy habits before and during pregnancy

The CDC recommends that everyone who could become pregnant get 400 micrograms (mcg) of folic acid daily, starting at least a month before conception and continuing in pregnancy, to help prevent neural tube defects (CDC: About Folic Acid). People who have had a pregnancy affected by a neural tube defect should talk to their doctor, as a higher dose is usually recommended. Also avoid alcohol and smoking, avoiding alcohol and smoking, and staying active as your provider advises. Ask your provider about vaccines, medicines and any existing conditions early on.

The calculators in detail

Due Date Calculator

Estimates a due date from the first day of your last period with Naegele's rule, adjusted for cycle length, and shows gestational age and trimester dates. Your provider will confirm the date, usually by ultrasound.

Ovulation Calculator

Estimates ovulation and the six-day fertile window for your next three cycles. It is a calendar estimate for people trying to conceive and must not be used as contraception.