Gestational age and due dates are estimates rather than exact predictions. In singleton pregnancies, gestation averages about 40 weeks (280 days) from the first day of the last menstrual period, corresponding to roughly 266 days from conception, with spontaneous term birth commonly occurring between about 37 and 42 weeks even after accurate early dating.

ACOG guidance highlights that first-trimester ultrasound using crown-rump length up to and including 13 6/7 weeks is the most accurate way to establish or confirm gestational age, with an error margin of about plus or minus 5-7 days. Ultrasound between 14 0/7 and 21 6/7 weeks is slightly less precise, about plus or minus 7-10 days, and third-trimester ultrasound is the least reliable for dating, with potential errors of roughly plus or minus 3-4 weeks. Late scans are generally not used to re-date a pregnancy that already has good early dating.

Dating method	Typical use	Approximate accuracy
Certain LMP only	When cycles are regular and recall is reliable, and no early scan or ART dating is available	Less precise, discrepancies of more than 5 days are common when checked against first-trimester ultrasound
First-trimester ultrasound (CRL up to 13 6/7 weeks)	Preferred basis for assigning the official obstetric EDD	Approximately plus or minus 5-7 days
Ultrasound 14 0/7 to 21 6/7 weeks	Used when no earlier scan is available	Approximately plus or minus 7-10 days
Ultrasound 28 weeks or later	Primarily for growth and wellbeing, dating only when no earlier data exist	Least accurate, dating error may be 2-3 weeks or more

Robinson and Fleming's evaluation of sonar crown-rump length measurements demonstrated that early CRL can estimate gestational age reproducibly and underpins many modern first-trimester CRL charts. For pregnancies conceived with assisted reproduction, ACOG recommends deriving gestational age from the known fertilisation or embryo age rather than from LMP alone.

This tool applies these standard calendar assumptions to convert between LMP, gestational age on a known date, conception or fertilisation date and an EDD. It assumes a singleton intrauterine pregnancy and does not perform ultrasound biometry. Ultrasound findings should first be translated into a best obstetric EDD or gestational age before using the calculator. Accuracy is reduced when LMP is uncertain, cycles are markedly irregular or there has been recent hormonal contraception or breastfeeding. In these situations treat calendar-based outputs as approximate and follow local policies for assigning, documenting and, if necessary, revising the official EDD.

Estimating pregnancy due date and gestational age