Due Date Calculator

How Is Your Due Date Calculated?

A pregnancy due date is an estimate of when your baby will arrive, typically calculated as 280 days (40 weeks) from the first day of your last menstrual period. This method, known as Naegele's rule, has been the standard in obstetric practice since the 19th century. The formula assumes a 28-day cycle with ovulation occurring on day 14: though in reality, cycle lengths vary significantly between women. Enter the first day of your last period in the calculator above to get your estimated due date, current week of pregnancy, and trimester timeline.

Calculating Due Date from Your Last Menstrual Period

The last menstrual period (LMP) method is the most widely used approach to calculate a due date. Take the first day of your most recent period, add 7 days, subtract 3 months, and add 1 year. If your last period started on March 10, your estimated due date would be December 17. This calculation works best for women with regular 28-day cycles. If your cycle runs longer or shorter, the estimate shifts accordingly: a 35-day cycle means ovulation typically happens around day 21 rather than day 14, pushing the due date about one week later than the standard LMP calculation would suggest.

Due Date from Conception or Ovulation Date

If you know exactly when you conceived or ovulated, you can calculate your due date with greater precision. Conception-based dating adds 266 days (38 weeks) from the date of fertilization. This method is more accurate than LMP dating because it removes the uncertainty of cycle length variation. Women who track ovulation through basal body temperature, ovulation predictor kits, or cervical mucus monitoring often have a reliable conception date. The calculator above accepts both LMP and conception dates so you can use whichever information you have available.

IVF Due Date Calculation

For pregnancies conceived through in vitro fertilization, the due date calculation uses the embryo transfer date rather than a last menstrual period. An IVF due date is calculated by adding 266 days to the conception date, then adjusting for the age of the embryo at transfer. For a Day 3 embryo transfer, add 263 days to the transfer date. For a Day 5 blastocyst transfer, add 261 days. IVF due dates tend to be more accurate than natural conception estimates because the exact fertilization date is known. If you underwent a frozen embryo transfer, the same calculation applies based on the transfer date and embryo age at freezing.

How Accurate Are Due Date Estimates?

Only about 4-5% of babies arrive on their exact estimated due date. The reality is that a healthy full-term delivery can happen anywhere between 37 and 42 weeks of pregnancy. About 60% of babies are born within one week of the due date, and roughly 90% arrive within two weeks on either side. First-time mothers tend to deliver slightly later than the estimate, while women who have given birth before may deliver a few days earlier. Your due date is best understood as the center of a probable delivery window, not a precise appointment.

Ultrasound Dating vs LMP Calculation

First-trimester ultrasounds, performed between 8 and 13 weeks, are the most reliable method for confirming or adjusting a due date. During this window, embryo size correlates tightly with gestational age because growth rates are nearly universal in early pregnancy. An ultrasound measurement that differs from the LMP-based date by more than 5-7 days typically overrides the calculated estimate. Later ultrasounds become less accurate for dating because babies grow at increasingly individual rates as pregnancy progresses. A third-trimester ultrasound can be off by up to three weeks in either direction.

Understanding Your Pregnancy Timeline

Pregnancy is divided into three trimesters, each marked by distinct developmental milestones. The first trimester spans weeks 1 through 12 and is when all major organs begin forming. Morning sickness, fatigue, and breast tenderness are common. The second trimester, weeks 13 through 27, is often considered the most comfortable period: energy returns, nausea typically fades, and you begin feeling fetal movement around week 18 to 22. The third trimester, weeks 28 through 40, brings rapid fetal weight gain, increased physical discomfort, and preparation for labor. Knowing which trimester you are in helps you anticipate symptoms and schedule appropriate prenatal tests.

Week-by-Week Pregnancy Tracking

Once you have your due date, tracking your pregnancy week by week gives context to each stage of development. At week 6, the heart begins beating. Week 12 marks the end of the first trimester and the point at which miscarriage risk drops significantly. By week 20, the halfway point, most women have their anatomy scan ultrasound. Week 28 begins the third trimester and the point of viability outside the womb. Week 37 is considered early term, and week 39 is full term. Each milestone carries specific recommendations for nutrition, exercise, testing, and preparation that your healthcare provider will guide you through.

What to Do If Your Due Date Changes

It is common for your due date to be adjusted during pregnancy, especially after a first-trimester ultrasound reveals a difference from the LMP-based calculation. A revised date does not mean anything is wrong: it simply reflects more accurate information about when conception actually occurred. If your date changes, update your pregnancy tracking apps and remind your healthcare team so that all future assessments, including growth scans, gestational diabetes screening timing, and labor induction decisions, are based on the most accurate timeline. The calculator above can be re-run with updated information at any point during your pregnancy.

Common Due Date Myths

Many people believe that first babies always come late, but research shows first-time mothers deliver an average of just one to three days past their due date: a difference that falls within normal statistical variation. Another common misconception is that the due date is based on when you had intercourse. In reality, sperm can survive up to five days in the reproductive tract, and ovulation timing drives conception, not the date of intercourse. The idea that a full moon triggers labor is also unsupported by evidence: multiple large-scale studies have found no correlation between lunar cycles and birth rates. Your due date is a medical estimate based on gestational age, not folklore.

Frequently asked questions