Enter any available dates and the rest will be calculated. Includes corrected age for preterm infants.
Fill in what you know — the calculator will derive the rest.
Gestational age (GA) is calculated from the first day of the mother's last menstrual period (LMP) using Naegele's rule. The estimated due date (EDD) is LMP + 280 days (40 weeks). GA at any given date equals the number of days from LMP to that date, expressed in weeks and days.
For example, if the LMP was January 1 and today is March 15, that is 73 days, or 10 weeks and 3 days gestational age. First-trimester ultrasound dating may be used to adjust GA if it differs from LMP-based dating by more than 5-7 days.
Chronological age (also called postnatal age) is the time since birth. Corrected age (also called adjusted age) accounts for prematurity by subtracting the number of weeks born early from the chronological age.
Formula: Corrected age = Chronological age − (40 weeks − GA at birth). For example, a baby born at 28 weeks who is now 6 months chronological age has a corrected age of 6 months − 3 months = 3 months corrected. Most AAP guidelines recommend using corrected age for developmental milestones until 24 months for weight/height and 18 months for head circumference.
The estimated due date (EDD) is calculated as LMP + 280 days using Naegele's rule, which assumes a 28-day menstrual cycle with ovulation on day 14. This calculator also supports deriving EDD from other known data: if you enter a birth date and GA at birth, the EDD is back-calculated.
Modern obstetric practice often adjusts the EDD based on first-trimester ultrasound, which is the most accurate method (±5 days accuracy at 8-13 weeks). ACOG recommends that the EDD should be adjusted if the ultrasound date differs by more than 5 days in the first trimester.
AAP guidelines recommend correcting for prematurity until 24 months of age for weight and height assessments, and 18 months for head circumference. For developmental milestones, correction is generally applied until 2-3 years of age, depending on the degree of prematurity.
In practice: late preterm infants (34-36 weeks) may "catch up" by 12-18 months, while extremely preterm infants (< 28 weeks) may benefit from correction through 30-36 months. Beyond these ages, most children have caught up to their peers, and corrected age is no longer necessary.
Postmenstrual age (PMA) = gestational age at birth + postnatal age. For example, a baby born at 26 weeks who is now 4 weeks old has a PMA of 30 weeks. PMA is the standard age metric used in the NICU for clinical decision-making.
PMA is used to guide: caffeine therapy for apnea of prematurity (typically until 34-36 weeks PMA), retinopathy of prematurity screening (starting at 31 weeks PMA or 4 weeks postnatal age), Fenton growth chart plotting, and discharge readiness assessments.