SamCalculator

Due Date Calculator

Estimate your baby's expected delivery date using last period, ultrasound, conception date, or IVF transfer details.

Evidence-Based EstimationMobile FriendlyInstant ResultsEducational Use Only

Last reviewed May 2026 · Only ~5% of babies arrive on their estimated due date — treat this as a window, not a deadline.

Estimate based on

Day 1 of your most recent menstrual period.

Typical interval from one period's first day to the next. Default 28 days.

How Due Dates Are Calculated

A pregnancy due date — clinically called the estimated due date (EDD) — marks the end of 40 weeks of gestational age. Gestational age is measured from the first day of your last menstrual period (LMP), not from conception. So at the moment of fertilisation you're already counted as about 2 weeks pregnant.

This calculator implements four medically accepted dating methods, each best suited to a different starting point:

LMP (Naegele's Rule)

Due date = LMP + 280 days, then adjusted up or down for cycle lengths other than 28 days.

Ultrasound

Gestational age at the scan is back-projected to LMP, then forward to a 40-week due date. Most accurate in weeks 6–14.

Conception date

Due date = conception + 266 days. Useful when you have a documented ovulation, IUI, or LH-monitored cycle.

IVF transfer

Due date = transfer date + 263 / 261 / 260 days depending on whether a Day 3, Day 5, or Day 6 embryo was transferred.

Only about 4–5% of babies are born on their EDD. The medically meaningful range is the term window: 37 weeks 0 days through 41 weeks 6 days. Anything inside that window is normal.

LMP vs Ultrasound Dating

LMP-based dating using Naegele's Rule assumes a regular 28-day cycle with ovulation on day 14. It's simple and historically dominant, but it depends on three things being true: you remember the LMP accurately, your cycles are roughly 28 days, and you weren't on hormonal contraception during the cycle.

A first-trimester ultrasound (6–14 weeks) measures crown-rump length, which is tightly correlated with gestational age. ACOG considers it the most accurate non-IVF dating method, with a typical precision of ±5–7 days. When LMP and a first-trimester scan disagree by more than that window, the EDD is re-dated to the ultrasound. After 22 weeks, growth between individual fetuses diverges and ultrasound dating becomes less reliable — the original LMP-based date is usually preserved.

IVF Pregnancy Dating Explained

IVF pregnancies have the most precise due dates of any conception method because the day of fertilisation is laboratory-known. The calculator adjusts for the embryo's developmental stage at transfer:

  • Day 3 embryo: due date = transfer date + 263 days. The embryo is at the cleavage stage with ~8 cells.
  • Day 5 blastocyst: due date = transfer date + 261 days. The most common transfer stage; embryo has reached the blastocyst stage.
  • Day 6 blastocyst: due date = transfer date + 260 days. An expanded or slower-to-blastocyst blastocyst, transferred one day later.

Each formula equals 280 days from a back-projected LMP minus the embryo's age at transfer. So a Day 5 IVF baby and a naturally conceived baby implanting on the same calendar day have the same due date.

What Makes Due Dates Change

It's normal for an EDD to shift by a few days during prenatal care — especially after the first dating scan. The most common reasons:

  • Cycle length differs from 28 days. Longer cycles delay ovulation, shorter cycles bring it forward. The calculator's LMP mode applies this adjustment directly.
  • LMP is uncertain. Postpartum, on hormonal contraception, or with irregular periods, the recalled LMP can be off by a week or more.
  • First-trimester ultrasound disagrees. If the scan's crown-rump length implies a gestational age more than 5–7 days different from LMP, the EDD is re-dated.
  • Conception happened later in the fertile window. Sperm can survive ~5 days; the calculator assumes ovulation-day fertilisation, but actual conception can be 1–4 days earlier.

Understanding Pregnancy Trimesters

Pregnancy is conventionally divided into three trimesters based on weeks of gestational age:

First trimester

Weeks 1–13

Organ formation. Highest miscarriage risk in weeks 1–12. Window for nuchal-translucency / NIPT screening.

Second trimester

Weeks 14–27

Anatomy ultrasound around week 20. Most parents start to feel fetal movement (quickening) between 16–22 weeks.

Third trimester

Weeks 28–40

Glucose tolerance test, Rh-negative anti-D injection if needed, group B strep test around week 36, full term from week 39.

ACOG further subdivides the term window: early term is 37w 0d–38w 6d, full term is 39w 0d–40w 6d, late term is 41w 0d–41w 6d, and post-term is 42w 0d and beyond. Babies born in the full-term window have the best long-term outcomes.

Frequently Asked Questions

Due date calculators are estimates — only about 4–5% of babies are actually born on their estimated due date. LMP-based estimates assume a regular 28-day cycle with day-14 ovulation, so cycle variation introduces several days of uncertainty. A first-trimester ultrasound is generally accurate to within ±5–7 days; IVF transfer dating is accurate to within ~1 day because the fertilisation day is known.

The classic Naegele's Rule adds 280 days (40 weeks) to the first day of the LMP. If your cycle is longer or shorter than 28 days, the calculator adjusts ovulation timing: a 32-day cycle ovulates later and pushes the due date back by 4 days, while a 24-day cycle pushes it forward by 4 days.

Yes. If a first-trimester ultrasound's gestational age differs from your LMP-based estimate by more than 5–7 days (ACOG threshold), the clinical due date is usually re-dated to the ultrasound. The first trimester is the most reliable dating window because fetal growth is tightly correlated with gestational age before week 14.

For IVF, the embryo's age at transfer is known precisely. The due date is computed from the transfer date plus a fixed offset that depends on the embryo's developmental stage: 263 days for a Day 3 transfer, 261 days for a Day 5 blastocyst, and 260 days for a Day 6 blastocyst. IVF gives the most accurate due dates of any method.

Pregnancy lasts about 266 days from conception (fertilisation) to birth, which is 280 days from the LMP minus the ~14-day pre-ovulation window. So if the conception date is known — for example from a documented ovulation day or a single IUI — the due date is simply conception date + 266 days.

The calculator's LMP mode lets you set cycle length from 22 to 44 days. Cycles longer than 28 days shift the due date later; cycles shorter than 28 days shift it earlier. The adjustment is one day for each day of difference from 28. If you have an irregular cycle, an early ultrasound gives a much more reliable due date than LMP.

Gestational age is measured from the first day of your last menstrual period — so at conception you're already about 2 weeks pregnant. Foetal age starts at conception and is roughly 14 days less than gestational age. Almost all obstetric care and milestone tracking uses gestational age, so this calculator reports week counts in gestational age.

ACOG defines four term categories. Early term is 37 weeks 0 days through 38 weeks 6 days. Full term is 39 weeks 0 days through 40 weeks 6 days. Late term is 41 weeks 0 days through 41 weeks 6 days. Post-term is 42 weeks 0 days and beyond. Babies delivered in the full-term window have the best long-term outcomes.

The first trimester runs from week 1 (LMP) through the end of week 13. The second trimester runs from week 14 through the end of week 27. The third trimester runs from week 28 through delivery — typically around week 40. Each trimester has distinct fetal-development milestones and a different set of prenatal screening tests.

No. This calculator provides educational estimates only and does not replace medical advice from your healthcare provider. Your obstetrician, midwife, or family-medicine provider will refine your due date based on dating ultrasounds, fundal-height measurements, and clinical context that no online tool can replicate.

References

  1. 1. American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology, 2017. acog.org
  2. 2. Centers for Disease Control and Prevention (CDC). Fertility and Pregnancy. cdc.gov/reproductive-health
  3. 3. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). Pregnancy & Prenatal Care. nichd.nih.gov

Related Calculators