Gestational Age Calculator | Fetal Age from LMP & Clinical

A gestational age calculator measures how far along a pregnancy has progressed, counted from the mother's last menstrual period (LMP). Unlike postnatal chronological age — which starts at birth — gestational age starts roughly two weeks before conception, because the medical convention counts from the first day of the LMP. This guide gives you an interactive terminal-style calculator, the LMP and clinical (ultrasound) formulas, the three-trimester breakdown, and the medical-versus-legal definition of the age of an unborn child.

$ awaiting LMP input...
$ enter the mother's last menstrual period date below▊

Gestational Age Calculator: LMP Method vs Clinical/Ultrasound Method

There are two accepted ways to determine gestational age, and the gestational age calculator above supports both. The LMP method asks the mother for the first day of her last menstrual period and counts forward; it is simple but assumes a textbook 28-day cycle with ovulation on day 14. The clinical method measures the fetus directly — most commonly the crown-rump length (CRL) in the first trimester — and is more accurate when the LMP is uncertain or the cycle is irregular. By definition, gestational age means the age of an unborn child calculated from the LMP, while the clinical age of the unborn child is calculated from direct fetal measurement.

Gestational Age Formula & Interactive LMP Calculator

The mother lmp to calculate age of unborn child workflow is the core of the LMP method. The formula is straightforward: gestational age in days equals today's date minus the LMP date, then converted to weeks and remaining days (7 days per week). For the estimated due date (EDD), Naegele's rule adds 280 days to the LMP — reflecting the average 40-week gestation. When the cycle length differs from 28 days, the EDD shifts by the difference (a 35-day cycle pushes ovulation later, so 7 days are added to the EDD).

Toggle the "Adjust for cycle length" switch in the terminal above to enter your actual cycle length, and the EDD recalculates instantly. Toggle "Ultrasound CRL correction" and the calculator switches to the Hadlock first-trimester approximation GA(weeks) ≈ CRL(mm)/10 + 6.5 — the confidence bar turns green because ultrasound-anchored dating is more precise than LMP alone.

Computation Logic Decomposition Table

Three Trimesters & Fetal Development Milestones

Gestational age is conventionally grouped into three trimesters, each spanning roughly 13–14 weeks. The gestation age means the age of an unborn child calculated from the LMP, and these trimester boundaries help clinicians stage prenatal care and developmental screening.

The Clinical Age of the Unborn Child: Medical vs Legal Definition

The clinical age of the unborn child is calculated from direct fetal measurement — most often the crown-rump length in the first trimester, or biparietal diameter and femur length later on. In medicine, this clinical age is used to confirm or correct the LMP-based estimate and to time interventions like the anatomy scan or steroid administration for fetal lung maturation.

In law, the "age of an unborn child" carries very different consequences. Some jurisdictions define gestational age thresholds for viability (often 22–24 weeks) that trigger legal protections or restrictions. Gestational age also appears in statutes governing fetal homicide, stillbirth certification, and abortion cutoffs — contexts where a week-level difference can change the legal outcome. Because the gestational age means the age of an unborn child calculated from the LMP, while clinical age is measured from the fetus itself, the two can diverge by a week or more, and courts typically defer to the ultrasound-dated clinical age when the LMP is contested.

From Gestational to Corrected Age: Bridging Prenatal and Postnatal

Gestational age at birth also sets up the postnatal concept of "corrected age" — the developmental age used for premature infants. If a baby is born at 32 weeks gestational age (8 weeks early), the corrected age subtracts those 8 weeks from the chronological age until the child turns two. So a 6-month-old born at 32 weeks has a corrected age of 4 months, which is the age against which developmental milestones should be assessed. This is why gestational age and postnatal chronological age form a continuous chain: the gestational age calculator feeds directly into the corrected-age calculation used in pediatric follow-up.

To continue that chain after birth, see our guide on how to calculate a child's chronological age and the child's age in weeks calculator for the postnatal period.

Conclusion

The gestational age calculator above converts the mother's LMP into precise gestational weeks and days, classifies the trimester, and estimates the due date via Naegele's rule — with an optional ultrasound CRL correction for higher confidence. Whether you use the LMP method or the clinical method, remember that gestational age is measured from the last menstrual period, not from conception, and that ultrasound dating supersedes LMP dating when the two diverge by more than a week in the first trimester. For any clinical decision, always confirm the calculated result with your obstetric provider.