Gestational Age Calculator

What Is Gestational Age?

Gestational age refers to the age of a pregnancy counted from the first day of the last menstrual period (LMP), measured in weeks and days. By convention, gestational age begins approximately two weeks before conception actually occurs (at ovulation). A full-term pregnancy is 40 weeks (280 days) from the LMP. Accurate gestational age determination is critical for timing prenatal screening tests, assessing fetal growth, planning delivery, and managing preterm or post-term pregnancies.

Naegele's Rule

Naegele's rule is the standard method for estimating the due date (EDD): add 280 days (40 weeks) to the first day of the LMP. An alternative formulation is to add 7 days to the LMP date and subtract 3 months. This rule assumes a regular 28-day menstrual cycle with ovulation on day 14. For women with longer or shorter cycles, the EDD can be adjusted by adding or subtracting the difference from 28 days. Despite its simplicity, Naegele's rule remains a useful clinical tool when ultrasound dating is unavailable.

Ultrasound vs. LMP Dating

First-trimester ultrasound (measuring crown-rump length) is the most accurate method for dating a pregnancy, with a margin of error of ±5–7 days. ACOG recommends that if the ultrasound-based EDD differs from the LMP-based EDD by more than 7 days in the first trimester, 10 days in the second trimester, or 21 days in the third trimester, the ultrasound date should be used for clinical management. Once a dating method is established, it should not be changed later in pregnancy.

Key References

• ACOG Committee Opinion No. 700. Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.
• Whitworth M, et al. Ultrasound for fetal assessment in early pregnancy. Cochrane Database Syst Rev. 2015;(7):CD007058.