Pregnancy Due Date Calculator

How Due Dates Are Calculated

An estimated due date marks 40 weeks, or 280 days, of pregnancy counted from a starting reference point that depends on which dating method is used. The most common method is Naegele's Rule, the standard formula used by clinicians: it adds 280 days to the first day of the last menstrual period (LMP), assuming a typical 28-day cycle with ovulation around day 14. If a person's cycles run longer or shorter than 28 days, adjusting for average cycle length shifts the estimated ovulation date, and with it, the due date.

If someone knows their conception date instead — from ovulation tracking, a fertility monitor, or a single instance of intercourse — the calculator adds 266 days, since conception typically occurs about two weeks after the LMP. For pregnancies conceived through in vitro fertilization (IVF), the due date is calculated from the embryo transfer date: 263 days are added for a Day 3 transfer, and 261 days for a Day 5 (blastocyst) transfer, reflecting the embryo's known age at the time of transfer. Because the fertilization date is precisely documented in IVF, this method is considered one of the most reliable ways to date a pregnancy.

LMP Dating vs. Ultrasound Dating

LMP-based dating assumes a textbook 28-day cycle with ovulation on day 14, which doesn't hold true for everyone. Cycles that are naturally longer or shorter, irregular ovulation, or simply not remembering the exact first day of the last period can all throw off an LMP-based estimate by a week or more.

A first-trimester ultrasound, particularly one performed between about 8 and 13 weeks using the fetal crown-rump length, is widely regarded as the most accurate way to date a pregnancy, since early fetal growth is remarkably consistent regardless of cycle length. When an early ultrasound measurement differs from an LMP-based due date by more than roughly 5 to 7 days, a healthcare provider will typically re-date the pregnancy using the ultrasound instead. Once set from a first-trimester scan, a due date is usually not adjusted again later in pregnancy, since growth becomes more variable between individual babies as pregnancy progresses.

Why Only About 5% of Babies Are Born On Their Due Date

An estimated due date is the midpoint of a normal range, not a prediction of the exact day labor will start. Full-term pregnancies commonly deliver anywhere from 37 to 42 weeks, and healthy, spontaneous labor can begin at any point within that five-week window. Research on delivery timing has repeatedly found that only around 4 to 5% of babies actually arrive on their calculated due date, while the rest are born anywhere from a couple of weeks early to nearly two weeks after the estimated date.

This variability comes down to individual differences in how pregnancies progress: the exact trigger for labor isn't fully understood, and factors such as whether it's a first pregnancy, genetics, and overall health all play a role in timing. Rather than treating a due date as a deadline, it's more useful to think of it as the center of a broad, entirely normal range.

What To Do With Your Estimated Due Date

Once a due date is calculated, it's best used as a planning tool rather than a fixed appointment. It's a useful reference point for scheduling prenatal visits, tracking fetal growth milestones, planning parental leave, and knowing when a pregnancy reaches full term.

Share the calculated due date with a healthcare provider at the first prenatal appointment; they will confirm or adjust it based on a physical exam and, in most cases, a dating ultrasound. If a calculated due date and a provider's ultrasound-based estimate differ by more than about a week, the clinical dating should generally be trusted, since it accounts for factors a calculator can't, such as individual cycle variation and fetal growth pattern. This tool is designed to give a well-informed estimate to start the conversation, not to replace the medical care and monitoring a qualified provider gives throughout pregnancy.

Pregnancy Week-by-Week Milestones

Pregnancy is generally divided into three trimesters and marked by a handful of clinically significant milestones, each associated with a typical gestational age counted from the last menstrual period. These general milestones apply to most singleton pregnancies dated from LMP.

• Week 13 — End of the first trimester

  The risk of miscarriage drops substantially, and many people begin sharing the news with friends and family.

• Week 20 — Anatomy scan

  A detailed ultrasound checks fetal growth and organ development, and can often reveal the baby's sex.

• Week 24 — Viability

  This is generally considered the earliest point at which a baby born prematurely has a reasonable chance of survival with intensive medical care.

• Week 27 — End of the second trimester

  The third trimester begins, typically bringing more frequent prenatal visits.

• Week 37 — Full term

  A baby born at this point or later is considered full term, with fully developed lungs in most cases.

• Week 40 — Estimated due date

  This marks the end of a standard 280-day gestational count from the LMP.

• Week 42 — Post-term

  Pregnancies extending beyond this point are typically monitored closely or induced, per standard clinical guidance.

Due Date Calculator FAQ