Due date calculator by conception date

How this calculator works

A typical pregnancy lasts 266 days from conception — about 38 weeks. To estimate your due date, this tool simply adds 266 days to the conception date you enter. That figure comes from decades of obstetric research and is the same baseline that the American College of Obstetricians and Gynecologists uses in its dating guidance.

If your conception date is accurate — say, you tracked ovulation with LH strips or know your IVF transfer day — the result is reliable. The calculator runs all date math in UTC so a late-night click won't shift the answer by a day, and clamps any future or implausible dates to keep the result sensible.

When did I conceive vs. last period — what's the difference?

Doctors usually measure pregnancy from the first day of your last menstrual period (LMP), not from conception. That's because most people remember their last period more reliably than the exact day they ovulated. The catch is that ovulation — and therefore conception — typically happens about 14 days after the start of your last period.

That two-week gap is why a pregnancy is described as 40 weeks long (LMP) but only 38 weeks of actual fetal development (266 days from conception). If you're comparing what you see here with what your provider says, that's the difference: their number will usually be about two weeks higher.

Use this calculator when you genuinely know your conception date. If you don't, use the LMP-based due date calculator instead — that's the standard Naegele's-rule version your doctor uses.

Calculating due date for IVF transfers

IVF gives you the most precise dating possible because the day of transfer is recorded in your fertility clinic's chart. The formula adjusts for how old the embryo was when it was transferred:

• 5-day blastocyst transfer: due date = transfer date + 261 days (266 minus the embryo's 5-day age)
• 3-day cleavage-stage transfer: due date = transfer date + 263 days (266 minus 3)
• Frozen embryo transfer (FET): same math as a fresh transfer at the matching embryo age — what counts is the embryo's development stage on the day of transfer.

Switch the calculator above to “IVF transfer” and pick the matching embryo age. Because IVF transfer dates are exact, the resulting due date is often more accurate than dating by ultrasound — but your clinic will still confirm with an early scan.

For more on the biology of an IVF transfer day, the Mayo Clinic's IVF overview walks through each stage.

What if I don't know my exact conception date?

If you weren't tracking ovulation, a conception-date calculator isn't the right tool — your due date will be off by however many days your guess is wrong. In that case, calculate from your last menstrual period instead, which is what most providers and prenatal apps do by default.

A reasonable estimate: ovulation usually happens around 14 days before your next expected period, so if your cycles are regular, count back to that day. If your cycles are irregular or you're simply unsure, wait for your dating ultrasound — typically scheduled between weeks 8 and 12 — which will set your due date based on the embryo's actual size.

For the standard last-period method, use the LMP-based due date calculator — same Naegele's-rule formula your doctor uses. Earlier in the journey? The implantation calculator estimates your earliest reliable pregnancy-test date. And if you're curious about your baby's gender, the baby gender predictor by date tries three traditional methods side by side — strictly for fun, not medical.

Pregnancy week-by-week from conception

This table shows what's typically happening at each stage, measured both from conception (fetal age) and from your last menstrual period (gestational age). Your provider will usually quote the second number.

• Week 1–2 · Week 3–4 LMP

  1st trimester

  Fertilization, implantation, first missed period.

• Week 3–4 · Week 5–6 LMP

  1st trimester

  Heart begins to beat. Most home tests turn positive.

• Week 5–6 · Week 7–8 LMP

  1st trimester

  Major organs start forming. First prenatal visit usually scheduled.

• Week 7–8 · Week 9–10 LMP

  1st trimester

  Baby is officially called a fetus. Ultrasound shows movement.

• Week 10 · Week 12 LMP

  1st trimester

  End of first trimester. Miscarriage risk drops sharply.

• Week 14 · Week 16 LMP

  2nd trimester

  Baby's sex is often visible on ultrasound.

• Week 18 · Week 20 LMP

  2nd trimester

  Anatomy scan. Many parents feel first kicks (“quickening”).

• Week 22 · Week 24 LMP

  2nd trimester

  Viability milestone with NICU support.

• Week 26 · Week 28 LMP

  3rd trimester

  Third trimester begins. Glucose screening for gestational diabetes.

• Week 34 · Week 36 LMP

  3rd trimester

  Baby is considered “late preterm” if born now — usually fine.

• Week 37 · Week 39 LMP

  3rd trimester

  Full term. Most babies arrive in the next three weeks.

• Week 38 · Week 40 LMP

  3rd trimester

  Estimated due date. Only ~5% of babies arrive on this exact day.

Frequently asked questions

Sources and medical references

Every figure on this page is grounded in published guidance from major medical bodies. Where possible we link the deep page rather than the homepage so you can read the source yourself.

• ACOG — Methods for Estimating the Due Date — Committee Opinion (2017)
• Mayo Clinic — Fetal development: The 1st trimester
• NHS — Pregnancy due date calculator
• American Pregnancy Association — Pregnancy calculator and due date estimator
• Mayo Clinic — In vitro fertilization (IVF) — overview and process