Due Date Calculator

How your due date is calculated

Doctors and midwives start with Naegele’s rule: your due date is 280 days (40 weeks) after the first day of your last menstrual period (LMP). The old shortcut of “add one year, subtract three months, add seven days” gets you close, but it can be off by a few days because months have different lengths, so this calculator adds the 280 days directly. If your last period started on May 1, 2026, your due date is February 5, 2027; the shortcut would say February 8.

The rule assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter, ovulation shifts by the difference, and so does your due date. With the same May 1 period and a 32-day cycle, the estimate moves to February 9, 2027; with a 25-day cycle, to February 2.

If you know when you conceived, the due date is 266 days after conception. IVF dates are the most precise: add 263 days to a day-3 embryo transfer or 261 days to a day-5 blastocyst transfer. A day-5 transfer on May 20, 2026 also gives February 5, 2027.

How far along am I? Weeks and trimesters

Pregnancy is counted from the first day of your last period, not from conception, so you are already about 2 weeks “pregnant” when you conceive and about 4 weeks along when you miss a period. The calculator shows completed weeks plus days, the way your chart does: 25 weeks, 6 days means you are in week 26.

Trimesters follow the American College of Obstetricians and Gynecologists (ACOG): the first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks until birth.

Key prenatal dates week by week

The first prenatal visit usually happens around 8 to 10 weeks. Cell-free DNA screening (NIPT) is available from 10 weeks, and the nuchal translucency ultrasound is done between 11 and 13 weeks 6 days. The anatomy scan, often called the 20-week ultrasound, is scheduled between 18 and 22 weeks.

Glucose screening for gestational diabetes takes place between 24 and 28 weeks. The CDC recommends a Tdap vaccine during weeks 27 through 36 of every pregnancy, and an RSV vaccine at 32 through 36 weeks if those weeks fall between September and January. The group B strep (GBS) swab is done between 36 weeks 0 days and 37 weeks 6 days.

Most people see their provider every 4 weeks until 28 weeks, every 2 weeks from 28 to 36 weeks, and weekly after that. Your OB-GYN or midwife may schedule things differently based on your history, so treat the table as a planning guide.

How accurate is a due date?

Only about 4% of babies are born on their due date. ACOG defines early term as 37 weeks 0 days through 38 weeks 6 days, full term as 39 weeks 0 days through 40 weeks 6 days, late term as 41 weeks, and post-term as 42 weeks or later. A birth before 37 weeks is preterm.

An early ultrasound measures the baby’s crown-rump length and is the most accurate way to date a pregnancy. If it differs from your period-based date by more than about 5 to 7 days in the first trimester, your provider will usually change your due date to match the scan. After that, the corrected date is used for every appointment.