Due Date Calculator

Choose the dating method you have information for. If you know several, an early ultrasound is usually the most reliable, followed by an IVF transfer date, then the first day of your last period.

Estimation of due date

A due date, or estimated date of delivery, is the day on which a pregnancy reaches 40 weeks. By convention, pregnancy is counted from the first day of the last menstrual period (LMP), not from conception, because the start of a period is something most people can recall while ovulation usually passes unnoticed. In a typical 28-day cycle, ovulation and conception happen around two weeks after the LMP, so the baby's actual age is about two weeks less than the gestational age.

The traditional calculation is Naegele's rule, credited to the 19th-century German obstetrician Franz Naegele: add one year, subtract three months and add seven days to the first day of the last period, which is the same as adding 280 days. The rule assumes a 28-day cycle. If your cycles are longer, ovulation probably happened later, so the calculator moves the due date later by the same number of days; shorter cycles move it earlier.

When the conception date is known, for example from ovulation tracking, the due date is 266 days (38 weeks) after conception. With IVF the dates are precise: the due date is 266 days after the egg retrieval, which works out to 263 days after a 3-day embryo transfer and 261 days after a 5-day blastocyst transfer.

Ultrasound dating measures the embryo or fetus directly. In the first trimester the crown-rump length, the distance from the top of the head to the bottom of the torso, predicts gestational age to within about five to seven days. The American College of Obstetricians and Gynecologists (ACOG) considers a first-trimester ultrasound the most accurate way to establish or confirm the due date, and recommends changing an LMP-based date when the two differ by more than a set number of days, for example more than seven days between 9 and 14 weeks. Later scans are less precise for dating because babies grow at different rates, so a due date is normally not changed based on a late ultrasound alone.

Due date = first day of last period + 280 days + (cycle length − 28 days)

From a conception date add 266 days. From an IVF transfer add 266 days minus the embryo's age at transfer (3, 5 or 6 days). From an ultrasound, subtract the measured gestational age from the scan date to find the equivalent period date, then add 280 days.

Due date as a reference point

A due date is best understood as the middle of a window, not an appointment. Only about 4% to 5% of babies are born on their exact due date. Most arrive within two weeks either side of it, and first pregnancies tend to run a little longer on average.

ACOG and the Society for Maternal-Fetal Medicine define a term birth more precisely by weeks of pregnancy. Birth before 37 weeks is preterm. From 37 weeks 0 days through 38 weeks 6 days is early term, 39 weeks 0 days through 40 weeks 6 days is full term, 41 weeks 0 days through 41 weeks 6 days is late term, and 42 weeks or later is post term. Babies born at full term generally have the best health outcomes, which is why elective deliveries before 39 weeks are discouraged without a medical reason.

Even though it is an estimate, the due date is important. Care providers use it to schedule screening tests at the right time, such as first-trimester genetic screening, the anatomy scan at 18 to 22 weeks and glucose screening at 24 to 28 weeks. It also guides how a baby's growth is judged and helps decide when to consider inducing labor if a pregnancy continues past 41 weeks. If you are unsure of your dates, mention it at your first prenatal visit so an early ultrasound can be arranged.

Frequently asked questions

Sources

• ACOG – Methods for Estimating the Due Date
• ACOG – Women's Health
• NICHD – Pregnancy