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Due Date

The due date is the center of a window — not an appointment.

From the first day of your last period, add 280 days — a January 15 LMP lands on October 22. This calculator estimates the due date either way (period or conception), counts your current week, and names your trimester.

Why people use this

Expecting parents use it the day the test turns: LMP + 280 days, current week, trimester — and the truth that the due date is a window, not an appointment.

How it works

1. Choose whether you know your last period start or the conception date.
2. Enter the date.
3. The tool applies Naegele's rule: LMP + 280 days, or conception + 266 days.
4. Read the estimated due date, completed weeks, and current trimester.
5. Bring the estimate to your first appointment — your provider may adjust it by ultrasound.

Common questions

How accurate is a due date calculator?

Naegele's rule (LMP + 280 days) assumes a textbook 28-day cycle with ovulation on day 14 — real cycles vary, so first-trimester ultrasound is the gold standard and providers adjust the date when early ultrasound disagrees by more than about a week. Only about 4% of babies arrive on the exact due date; the honest framing is a window, with most spontaneous births landing between 39 and 41 weeks. Longer cycles shift ovulation (and the real due date) later; shorter cycles earlier. If your cycles run 35 days, mentally add a week to the estimate until your provider weighs in.

Why do doctors count pregnancy from the last period?

Historical practicality: the LMP is a date most people know, while conception almost always happens later (about day 14 of a 28-day cycle) and is rarely known exactly. So "40 weeks of pregnancy" includes roughly two weeks before conception occurred — at "4 weeks pregnant," the embryo is barely implanted. This is why the conception-date option in this calculator adds 266 days (280 − 14) instead. IVF pregnancies count from embryo transfer with the embryo's age added — a day-5 blastocyst transfer uses transfer + 261 days. Same 40-week math, better-known starting line.

What are the trimester milestones?

First trimester (weeks 1–13): implantation, organ formation, and the highest miscarriage-risk window — which is why many announce after week 12–13. Second (14–27): the anatomy scan at ~20 weeks, first felt movement around 18–22 weeks, and the viability milestone at ~24 weeks. Third (28–40): rapid growth, position checks, and the term definitions that matter — early term 37–38 weeks, full term 39–40, late term 41, post-term 42+. Many providers discuss induction by 41 weeks because risks to the baby climb gently past the due date. Each milestone maps to a week count from your LMP, which is exactly what the progress readout tracks.

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