How is a pregnancy due date calculated from the last period?

A common estimate counts 280 days from the first day of the last menstrual period and can shift that estimate when cycle length differs from 28 days.

How is an IVF due date estimated?

The embryo age and transfer date establish an ART-derived gestational age; a day-5 transfer is commonly calculated by adding 261 days.

Is the calculated date exact?

No. It is an estimate, and a qualified maternity clinician should establish or confirm the date used for care, often with first-trimester ultrasound.

Pregnancy Due Dates Calculator

Estimate a due date and pregnancy milestones from the first day of a last period, a known conception date, or a day-3 to day-6 IVF embryo transfer.

Written and checked by TheToolNet Editorial Team · Last reviewed: October 2, 2026 · Editorial method

Pregnancy care warning: confirm every date with a maternity professional.

This calculator gives a general estimate and may be inaccurate. It cannot confirm pregnancy, fetal health, paternity, conception timing, or when labour will begin. A doctor, obstetrician, midwife, fertility clinic, and appropriate ultrasound should determine the dates used for care. Seek urgent medical help for severe pain, heavy bleeding, fainting, breathing difficulty, seizures, or any symptom your care team has told you is urgent.

Estimate pregnancy due dates

How pregnancy due dates are estimated

The last-period method starts at the first day of the LMP and counts 280 days, or 40 weeks. This calculator shifts the estimate by the difference between the entered cycle length and 28 days. A known conception date uses 266 days. IVF calculations use the transfer date and embryo age; for example, a day-5 transfer adds 261 days.

The American College of Obstetricians and Gynecologists states that first-trimester ultrasound is the most accurate method to establish or confirm gestational age and that ART-derived gestational age should be used after IVF.

A due date is an estimate

Pregnancy is tracked from the LMP even though conception typically occurs later. The NHS due-date guidance describes pregnancy as normally lasting from 37 to 42 weeks from the LMP and advises speaking with a midwife or doctor when the date is unknown or uncertain.

This tool does not predict a baby's sex

Calendar methods, conception timing, and “gender prediction” formulas cannot reliably determine fetal sex. This page therefore does not include them. Ask your maternity professional which clinically appropriate tests are available and what they can establish.

Frequently asked questions

What if my cycles are irregular?

An LMP calculation becomes less dependable. Tell your clinician about irregular cycles and use the dating method they recommend.

Can I calculate the exact day I conceived?

No. A due date can provide an approximate conception date, but ovulation, fertilization, and implantation timing vary. IVF records are more specific but still require clinic interpretation.

Why might an ultrasound date differ?

Ultrasound uses measured development rather than an assumed ovulation day. Your clinician applies professional dating rules to decide whether the care date should change.