Pregnancy Due Date Calculator

A due date calculator gives you an estimate — only about 5% of babies are born on their exact due date. Most arrive within two weeks before or after. The LMP method assumes a 28-day cycle, so it can be off if your cycles are irregular. First-trimester ultrasound (before 14 weeks) is generally the most accurate method, typically within 5–7 days, and your provider may adjust your due date based on those measurements.

The most common method is Naegele's rule: count 280 days (40 weeks) from the first day of your last menstrual period (LMP). This assumes ovulation on day 14 of a 28-day cycle. You can also calculate from a known conception or ovulation date (add 266 days) or from an IVF transfer date. If your provider has already given you a due date — usually from an early ultrasound — that date is typically more accurate than LMP-based calculations.

Yes. Your healthcare provider may revise your estimated due date after a first-trimester ultrasound. If the ultrasound measurements put your due date more than 7 days away from the LMP-based estimate, most providers will adjust it. Due dates can also shift at later ultrasounds if fetal growth is significantly ahead or behind. Once set by first-trimester ultrasound, the date typically doesn't change again.

Ultrasound dating measures the baby's actual size — crown-rump length in the first trimester, head circumference and femur length later on — and estimates gestational age from that. If your cycle is longer or shorter than 28 days, or if ovulation happened later than expected, the ultrasound date will differ from the LMP-based date. When there's a gap of 7 or more days, most providers go with the ultrasound, since it reflects what's actually happening in the uterus.

Pregnancy is divided into three trimesters. The first trimester spans weeks 1–12 and covers the most rapid early development. The second trimester covers weeks 13–26 — most people feel better during this stretch, and the anatomy scan happens around week 20. The third trimester runs from week 27 until birth. Your baby is considered early term at 37–38 weeks, full term at 39–40 weeks, and post-term at 42 weeks or beyond.

A full-term pregnancy is 39 to 40 weeks and 6 days. "Early term" is 37 to 38 weeks and 6 days. "Late term" is 41 to 41 weeks and 6 days, and "post-term" is 42 weeks and beyond. The American College of Obstetricians and Gynecologists (ACOG) discourages elective delivery before 39 weeks because those final weeks matter for brain, lung, and liver development.

If your cycles are irregular, the LMP method is less reliable. Try adjusting the cycle length input in the calculator to better match your typical cycle — the math shifts accordingly. Even better, if you tracked ovulation using OPKs, basal body temperature, or a fertility app, use the Conception Date tab for a more accurate estimate. Your provider will likely confirm or revise your due date at your first ultrasound, which is the most reliable method when cycle data is uncertain.

For IVF pregnancies, add 263 days to a Day 3 embryo transfer date, or 261 days to a Day 5 (blastocyst) transfer date. This method is the most precise because the exact fertilization timing is known. Use the IVF / FET tab in the calculator above and select your embryo type. Your fertility clinic will confirm the estimated due date at your first monitoring ultrasound.

Close, but not exactly. Pregnancy is measured in weeks, not calendar months, which is why it can feel confusing. Forty weeks works out to roughly 9 months and 1 week, depending on how you count. The discrepancy comes from the fact that calendar months vary between 28 and 31 days, while gestational weeks are always exactly 7 days. Most providers use weeks as the standard unit because they're more precise.

Gestational age counts from the first day of your last menstrual period — about two weeks before conception actually occurs. Fetal age counts from the date of conception. When your provider says you are "10 weeks pregnant," they mean 10 weeks gestational age; the embryo itself is approximately 8 weeks old. All week-by-week pregnancy content, including the milestones and baby size comparisons on this page, use gestational age.

Most home pregnancy tests can detect hCG from the first day of a missed period — typically about 14 days after ovulation. Some early-detection tests claim accuracy up to 6 days before a missed period, but testing too early increases the chance of a false negative. For the most reliable result, wait until the day of your expected period. If you get a negative result but your period doesn't arrive, test again in 2–3 days.

Going past your due date is common — only about 5% of babies arrive on the exact date. Your provider will monitor you more closely after 40 weeks, typically with nonstress tests and biophysical profiles to check on the baby. Most providers will discuss induction between 41 and 42 weeks, as the risks of waiting increase after 42 weeks. Going past your due date does not mean something is wrong, but stay in close contact with your care team.