Gestational Age Calculator

The Gestational Age Calculator determines the precise age of a pregnancy in weeks and days based on the interval between the last menstrual period (LMP) and any given date. Gestational age is the universal language of obstetrics, used to communicate pregnancy timing between healthcare providers, schedule appropriate tests and screenings, and determine the viability and maturity of the developing fetus.

Gestational age is conventionally measured from the first day of the last menstrual period rather than from the date of conception. This means that at the time of conception, a pregnancy is already considered to be approximately 2 weeks old. While this may seem counterintuitive, this convention was established long before the biology of ovulation was well understood and remains the standard because it provides a consistent, easily identifiable starting point that does not require knowledge of the exact ovulation or conception date.

The clinical significance of precise gestational age determination cannot be overstated. In the first trimester, it determines the appropriate timing for initial ultrasound evaluation, nuchal translucency screening (11-13 weeks), and first-trimester combined screening for chromosomal abnormalities. The second trimester brings the anatomy scan (18-22 weeks), quad screen or cell-free DNA testing, and glucose tolerance testing (24-28 weeks). Third-trimester management, including Group B Streptococcus screening (35-37 weeks) and the decision about delivery timing, all depend on accurate gestational age.

The three trimesters of pregnancy are defined by gestational age: the first trimester encompasses weeks 1 through 13 (days 0-91 from LMP), the second trimester spans weeks 14 through 26 (days 92-182), and the third trimester extends from week 27 to delivery. Each trimester is associated with distinct patterns of fetal development, maternal physiological changes, and potential complications that require different clinical approaches.

Accurate gestational age is particularly critical when evaluating preterm labor risk, as management strategies differ significantly based on whether the fetus is at 24 weeks (periviable, requiring complex counseling), 28 weeks (very preterm but with good survival prospects with NICU care), 32 weeks (moderate preterm with generally favorable outcomes), or 34 weeks (late preterm with minimal morbidity expected). Even a few days of difference in gestational age can influence decisions about antenatal corticosteroid administration, tocolytic therapy, and delivery planning.

When there is a discrepancy between LMP-based gestational age and ultrasound-estimated age, the American College of Obstetricians and Gynecologists provides specific guidance on which estimate to use. If the first-trimester ultrasound differs from LMP dating by more than 5-7 days, the ultrasound date is preferred. For second-trimester discrepancies, the threshold is 10-14 days. These guidelines ensure that the most accurate gestational age estimate is used for clinical decision-making throughout pregnancy.

This calculator provides gestational age in the standard obstetric format of completed weeks and additional days (for example, 21 weeks and 4 days), along with total days from LMP and the current trimester designation. This format is how gestational age is documented in medical records and communicated between providers worldwide.