Due Date Calculator

Estimate a baby's due date from the last menstrual period, a known conception date, or an IVF transfer date. An LMP of January 1 gives a due date of October 8 — 280 days (40 weeks) later, per the standard obstetric rule.

Calculate from

First day of last period

This is an estimate based on standard obstetric dating rules (Naegele's rule for LMP — 280 days / 40 weeks — and 266 days from conception or IVF retrieval). Only about 5% of babies arrive on their exact due date. Your doctor's ultrasound-based estimate is more accurate; use this for planning purposes only, not medical advice.

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Why use Due Date Calculator?

Due dates are calculated from a fixed rule, not measured directly — which is worth remembering when the number feels very precise. Naegele's rule (LMP + 280 days) assumes a standard 28-day cycle with ovulation on day 14; if a cycle runs longer or shorter, the actual conception date shifts, and so does the true due date. That's why a provider's early ultrasound-based estimate, which measures the fetus directly, is generally considered more accurate than an LMP-only calculation once available. IVF dating sidesteps most of that uncertainty since the fertilization date is known precisely — the calculator adjusts by the embryo's age at transfer (3 or 5 days) rather than assuming a cycle. Whichever method you use, treat the result as a likely window rather than a fixed date: only about 5% of babies are born on their calculated due date, with the large majority arriving within two weeks of it either way. This tool is for planning purposes only and isn't a substitute for guidance from your healthcare provider.

Key benefits

Three dating methods: LMP, conception, and IVF transfer
Shows current gestational age and trimester alongside the due date
IVF mode accounts for day-3 vs day-5 transfer
Clear disclaimer that this is an estimate, not a diagnosis

Features

Naegele's rule: LMP + 280 days (40 weeks)
Conception-based: conception date + 266 days (38 weeks)
IVF-based: transfer date + (266 − embryo age in days)

Popular use cases

Estimating a due date before or between doctor visits
Tracking current week of pregnancy and trimester
Cross-checking a due date given by a provider or app

How to use Due Date Calculator

1
Pick a method
Last menstrual period, conception date, or IVF transfer date.
2
Enter the date
The relevant date for your chosen method.
3
Read your estimate
Due date, current gestational age, and trimester.

Frequently asked questions

How is a due date calculated from the last period?

Using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. This assumes a textbook 28-day cycle with ovulation on day 14 — actual cycles vary, so this is an estimate, and an early ultrasound is more precise.

How accurate is a due date estimate?

Only about 5% of babies are born on their exact estimated due date — most arrive within a 2-week window either side. Due dates are a statistical estimate of the most likely delivery window, not a prediction of the exact day.

How is an IVF due date calculated?

From the transfer date, using 266 days minus the embryo's age at transfer — so 266 − 5 = 261 days added for a day-5 (blastocyst) transfer, or 266 − 3 = 263 days for a day-3 transfer. IVF dating is generally considered more precise than LMP-based dating since the fertilization date is known exactly.

What determines the trimester?

First trimester is weeks 0-13, second is weeks 14-27, and third is weeks 28-40, based on current gestational age from your last menstrual period.

Last reviewed: August 2026

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