Gestational Age Calculator

Estimate gestational age in weeks and days, plus the estimated due date, from the last menstrual period, a conception date, an IVF transfer date, or an ultrasound dating scan.

Quick Facts

Naegele's rule
EDD = LMP + 280 days
Assumes a regular 28-day cycle; gestational age is counted from the LMP, not from conception.
IVF dating offset
Day-3 transfer = 2w3d, day-5 = 2w5d
Embryo age at transfer is fixed, so it converts directly to gestational age.
Term range (ACOG)
37w0d - 41w6d
Split into early, full, and late term; 42w0d or beyond is post-term.

Your Results

Calculated
Gestational age
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Weeks and days as of the selected date
Estimated due date
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40 weeks (280 days) from LMP-equivalent dating
Trimester
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1st, 2nd, or 3rd trimester
Days until due date
-
Negative means past the estimated due date

Ready

Choose a dating method and enter the reference date to calculate gestational age.

About the gestational age calculator

Gestational age is the standard way obstetric care measures how far along a pregnancy is. By convention it is counted in completed weeks and days from the first day of the last menstrual period (LMP) — not from the date of conception, which is typically about two weeks later. This calculator converts a dating reference point (LMP, conception, an IVF transfer date, or an ultrasound measurement) into that LMP-based gestational age, then projects the estimated due date (EDD) and the number of days remaining.

The formula: Naegele's rule

The classic method, Naegele's rule, sets the estimated due date 280 days (40 weeks) after the first day of the LMP, assuming a regular 28-day cycle. Gestational age on any later date is simply the number of days elapsed since the LMP, expressed as weeks and days (for example, 203 elapsed days is 29 weeks 0 days). If you instead know a conception or ovulation date, this calculator adds 14 days to convert it to LMP-equivalent dating before applying the same 280-day rule.

IVF and ultrasound dating

For IVF pregnancies, the embryo's age at transfer is fixed by the lab: a day-3 cleavage-stage embryo corresponds to a gestational age of 2 weeks 3 days on the transfer date, and a day-5 blastocyst corresponds to 2 weeks 5 days. For ultrasound dating, enter the gestational age (weeks and days) that was measured at the scan; the calculator then counts forward or backward from the scan date using the same day-based arithmetic. Early first-trimester ultrasound (roughly 8-13 weeks) is generally the most accurate single dating method, since fetal size varies little between individuals at that stage.

Trimesters and term labels

Pregnancy is commonly divided into three trimesters: weeks 0 through 13 (plus 6 days), weeks 14 through 27 (plus 6 days), and week 28 through delivery. From 37 weeks, ACOG's term definitions apply: early term (37w0d-38w6d), full term (39w0d-40w6d), late term (41w0d-41w6d), and post-term (42w0d or beyond).

When to consult a professional

This calculator performs standard date arithmetic; it does not replace clinical dating, which typically combines LMP with an early ultrasound and may be adjusted by a care provider. For any question about your specific pregnancy, timing of prenatal care, or discrepancies between dating methods, consult a licensed healthcare provider.

Frequently Asked Questions

How is gestational age calculated?
By convention, gestational age is counted in weeks and days from the first day of the last menstrual period (LMP), not from the date of conception. Naegele's rule estimates the due date as the LMP date plus 280 days (40 weeks), assuming a regular 28-day cycle. If dating instead starts from a known conception or ovulation date, IVF transfer date, or an ultrasound measurement, this calculator converts that reference point into the equivalent LMP-based gestational age before projecting the due date.
What is the difference between gestational age and conceptional age?
Gestational age is measured from the last menstrual period and is the standard used by obstetric care. Conceptional (or embryonic/fetal) age is measured from the estimated date of conception, which is typically about 14 days after the LMP in a standard cycle. Gestational age is therefore usually about two weeks ahead of conceptional age.
How are IVF and ultrasound dating handled?
For IVF, the gestational age on the transfer day is fixed by the embryo's age: a day-3 cleavage-stage transfer corresponds to 2 weeks 3 days of gestational age, and a day-5 blastocyst transfer corresponds to 2 weeks 5 days, both counted from the LMP-equivalent start. For ultrasound dating, you enter the gestational age (weeks and days) that was measured at the scan, and the calculator projects forward or backward from that scan date using the same 7-day-per-week arithmetic.
What do the trimester and term labels mean?
The pregnancy is commonly split into three trimesters: weeks 0 through 13 (plus 6 days), weeks 14 through 27 (plus 6 days), and week 28 onward. Once gestational age reaches 37 weeks, ACOG's term labels apply: early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks 0 days to 41 weeks 6 days, and post-term is 42 weeks 0 days or beyond.

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