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Pregnancy Due Date Calculator

What Is an Estimated Due Date?

An estimated due date (EDD), also called the estimated date of confinement (EDC), is the projected date when a pregnant person is expected to deliver their baby. It is typically calculated as 280 days (40 weeks) from the first day of the last menstrual period, assuming a standard 28-day cycle. However, because cycle lengths vary widely (21–45 days) and ovulation doesn't always happen on day 14, more personalized methods — including cycle-length adjustment, luteal-phase correction, IVF transfer dates, and early ultrasound measurements — significantly improve accuracy. Research shows that only about 4–5% of babies are born on their exact due date. About 80% arrive within 10 days of the EDD, and any birth between 37 and 42 weeks is considered within the normal range. Your due date is best understood as the center point of a delivery window, not a precise deadline.

How Due Dates Are Calculated

This calculator supports five estimation methods. The Last Menstrual Period (LMP) method uses Naegele's rule with adjustments for your personal cycle length and luteal phase: it calculates the likely ovulation date (cycle length minus luteal phase length), then adds 266 days to reach the due date. The Conception Date method adds 266 days to the known date of fertilization. For IVF transfers, the calculator accounts for the embryo's age at transfer (3, 5, or 6 days) and adds the appropriate number of days to reach 40 weeks gestational age — IVF dates tend to be the most precise since the exact fertilization timing is known. The Ultrasound method works backward from the gestational age determined during an early scan, and first-trimester ultrasounds (before 13 weeks) are considered the gold standard for dating accuracy. Finally, the Known Due Date (reverse) method back-calculates your conception date and LMP, useful for understanding your complete pregnancy timeline.

Frequently Asked Questions

How accurate is a due date calculator?

Due date calculators provide an estimate based on average gestational length (280 days from LMP). Accuracy depends on the method: first-trimester ultrasound is most accurate (±5–7 days), IVF dates are nearly as precise, and LMP-based calculation can be off by 1–2 weeks for irregular cycles. Only about 4–5% of babies arrive on the exact due date — most are born within a 10-day window around the EDD.

What's the difference between gestational age and fetal age?

Gestational age is counted from the first day of your last menstrual period (LMP), which is about 2 weeks before conception actually occurs. Fetal age (embryonic age) is counted from the actual date of conception. So at '8 weeks pregnant' (gestational age), the embryo is actually about 6 weeks old. Medical professionals almost always use gestational age.

Can my due date change during pregnancy?

Yes. If an early ultrasound (before 13 weeks) shows a gestational age that differs from your LMP-based estimate by more than 7 days, your healthcare provider may adjust your due date. According to ACOG guidelines, first-trimester ultrasound measurements are the most reliable dating method.

How is an IVF due date calculated?

IVF due dates are calculated from the embryo transfer date. For a Day 5 blastocyst, 261 days are added to the transfer date. Day 3 embryos add 263 days, and Day 6 blastocysts add 260 days. IVF calculations are among the most accurate because the exact fertilization date is known.

What does 'full term' vs 'early term' mean?

ACOG defines: Preterm (before 37 weeks), Early Term (37–38 weeks), Full Term (39–40 weeks), Late Term (41 weeks), Post-Term (42+ weeks). Full term is the ideal window with the best health outcomes.

Does cycle length really affect the due date?

Yes, significantly. The standard 280-day calculation assumes a 28-day cycle with ovulation on day 14. A 35-day cycle means ovulation around day 21 — shifting your due date by 7 days. This calculator adjusts for both cycle length and luteal phase for better accuracy.

What is the Chinese Gender Prediction chart?

A traditional tool reportedly over 700 years old that claims to predict baby's sex based on the mother's lunar age and conception month. Scientific studies show it's about 50% accurate — the same as a coin flip. It's included here as a fun tradition, not a medical tool.

What is a 'dating ultrasound' and when should I get one?

A dating ultrasound (typically 7–12 weeks) measures the embryo's crown-rump length to determine gestational age. It's the most accurate dating method with ±5–7 days margin of error. ACOG and NHS both recommend it in the first trimester, especially for irregular cycles.

What happens if I go past my due date?

About 50% of first-time mothers deliver after 40 weeks. At 41 weeks, your provider increases monitoring. By 42 weeks, most guidelines (ACOG, NICE) recommend discussing induction as risks increase from declining placenta function and reduced amniotic fluid.

Can twins have a different due date?

Twin pregnancies use the same initial calculation, but expected delivery is earlier — around 36–37 weeks on average. Your provider may recommend delivery between 36–38 weeks depending on whether twins are identical (monochorionic) or fraternal (dichorionic).