Pregnancy Due Date Calculator

The most widely used method for estimating a due date is Naegele's Rule, named after Franz Karl Naegele, a German obstetrician who published the formula in 1806. The rule is straightforward: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This calculation assumes a regular 28-day cycle with ovulation on day 14. For cycles that differ from 28 days, the due date shifts accordingly — a 35-day cycle pushes the due date 7 days later, while a 21-day cycle moves it 7 days earlier.

Despite the precision of the formula, due dates are estimates, not appointments. Research consistently shows that only about 4% of babies arrive on their exact calculated due date. Roughly 80% of births occur within two weeks before or after the due date, meaning the true delivery window spans from about 38 weeks to 42 weeks. The due date is best understood as the midpoint of a statistically normal delivery range, not a fixed deadline.

There is an important distinction between gestational age and fetal age(sometimes called fertilization age). Gestational age counts from the first day of the LMP — which means it includes roughly two weeks before conception even occurred. Fetal age counts from fertilization. Doctors and ultrasound reports always use gestational age, which is why a positive pregnancy test at "4 weeks pregnant" describes an embryo that is only about 2 weeks old biologically. This terminology gap can be confusing but is universal in obstetric practice.

Ultrasound dating is the most accurate method for confirming gestational age, particularly in the first trimester. A measurement called crown-rump length (CRL) — the distance from the top of the embryo's head to its bottom — can date a pregnancy to within ±5–7 days when performed between 8 and 13 weeks. After the first trimester, ultrasound measurements become progressively less precise because fetal size varies more widely among individuals. A second or third trimester scan may have a margin of error of ±2–3 weeks. If an early ultrasound date differs from the LMP date by more than 7 days, most providers will adjust the official due date to match the ultrasound.

You might wonder: why do doctors use LMP as the starting point when conception actually happens about two weeks later? The answer is practicality. Most people know when their last period started. Conception date is often uncertain even when the approximate timing is known, because sperm can survive in the reproductive tract for up to 5 days. LMP provides a reliable, consistently defined reference point that every patient can usually report accurately.

Understanding what's happening at key milestones helps put the timeline in context. At Week 8, the embryo officially becomes a fetus — major organ systems have begun forming, and the heart has been beating since around week 6. By Week 12, the nuchal translucency (NT) scan can assess risk for chromosomal conditions such as Down syndrome. The Week 18–20 anatomy scan is a comprehensive ultrasound examining all major fetal structures, and fetal sex can typically be determined at this point if desired. Week 24 is considered the threshold of viability — survival rates at this gestational age range from 50–70% with intensive NICU care, improving significantly with each additional week. At Week 28, rapid brain development accelerates and providers often recommend beginning kick counting: 10 fetal movements within a 2-hour window is the standard guideline for reassurance.

Two important screening tests fall within specific windows. The glucose tolerance test (GTT) for gestational diabetes is typically performed between 24 and 28 weeks. The Group B Streptococcus (GBS) culture is collected between 36 and 37 weeks to determine whether IV antibiotics during labor are indicated. Missing these windows can require rescheduling, so noting them on your calendar early is worthwhile.

Knowing the signs of preterm labor — before 37 weeks — is equally important. These include regular contractions (even if painless), persistent lower back pain, pelvic pressure, or a change in vaginal discharge. Preterm labor is not always painful, so timing contractions even when they feel mild is recommended if they seem regular.

Finally, terminology around "term" was formally updated in 2013. Early term now refers to 37 weeks 0 days through 38 weeks 6 days. Full term is 39 weeks 0 days through 40 weeks 6 days. Late term covers 41 weeks 0 days through 41 weeks 6 days, and post-term begins at 42 weeks. Before 2013, anything from 37 weeks onward was called "term," but research showed that babies born at 37–38 weeks have meaningfully higher rates of breathing difficulties, feeding problems, and NICU admissions compared to those born at 39+ weeks — even though they were previously grouped together.

This calculator is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for guidance specific to your pregnancy.