Research-based pregnancy dating

Pregnancy Week Calculator

Use one source of pregnancy dating information or compare as many as you have to calculate completed pregnancy weeks and days today or on another calendar date.

How many weeks pregnant am I?

Which information is most useful?

One source is enough. If you have more, select and enter as many as you want. We compare them and use the strongest available source—we do not average due dates.

Due date established by your doctor or midwife
IVF embryo transfer or known fertilization date
First-trimester dating ultrasound report
A certain last period with regular cycles
Clinically tracked or well-tracked ovulation
Second-trimester ultrasound report
Later or less-certain sources, including uncertain periods, app predictions, IUI date alone, or third-trimester ultrasound

Your care team's established due date always remains authoritative.

Select every source you want to use

Select just one if that is all you have.

A due date from my doctor or midwife IVF, embryo transfer, fertilization, retrieval, or IUI An ultrasound report My last menstrual period An ovulation date

Calculate pregnancy week on

Your pregnancy timeline

Result

Estimated due date
Pregnancy week
Estimated fertilization
Plausible intercourse window

Key dates

Milestone	Date

How this calculator works

Pregnancy is conventionally dated from an estimated menstrual starting point, about two weeks before fertilization. A due date establishes that gestational timeline; it is not a promise that birth will happen on that day.

Dating methods and formulas

Clinician due date: used unchanged.
Last menstrual period: 280 days, adjusted by the difference between the usual cycle length and 28 days.
Ovulation or fertilization: 266 days.
Ultrasound report: the report's due date, or the scan date minus its reported gestational age and then plus 280 days.
Embryo transfer: 266 days from estimated fertilization, accounting for embryo age at transfer.
Egg retrieval or IUI: the event date is used as a conventional ART dating anchor. It is not presented as a known fertilization date.

Accuracy and uncertainty

First-trimester crown–rump length ultrasound is the most accurate routine ultrasound dating method. LMP calculations assume reliable dates and reasonably regular cycles. Even when ovulation is known, natural pregnancy length varies substantially.

Research and clinical guidance

This educational calculator does not establish or revise a clinical due date. Confirm pregnancy dating and care decisions with your maternity care professional.