Gestational age calculator for preterm infants
Enter any known dating anchors: LMP, estimated due date, birth date, gestational age at birth, or a reference date. The calculator derives the preterm age labels that matter for NICU, growth, and follow-up documentation.
Last reviewed: 2026-04-28
Use the best obstetric estimate
What this page calculates
The calculator uses 40 weeks as the term reference and derives the dating fields that can be calculated from the values entered. It works for prenatal dating, preterm birth documentation, and follow-up visits.
- EDD is 40 weeks after LMP, or back-calculated from birth date plus gestational age.
- Postmenstrual age is gestational age at birth plus postnatal age.
- Corrected age subtracts the prematurity gap from chronological age.
- Day of life is one-based: the date of birth is day of life 1.
Preterm classification
The page classifies gestational age at birth into extremely preterm, very preterm, moderate preterm, late preterm, term, or post-term bands. Use local policy for any operational cutoffs tied to screening or discharge.
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References
FAQs
It derives LMP, estimated due date, gestational age at birth, postmenstrual age, chronological age, corrected age, day of life, and a preterm classification when enough dates are entered.
Postmenstrual age is gestational age at birth plus postnatal age. NICU and preterm growth workflows often use PMA rather than chronological age alone.
Corrected age subtracts the prematurity gap from chronological age: corrected age = chronological age minus the difference between 40 weeks and gestational age at birth.
No. Use the established obstetric best estimate when available. This calculator is for documentation, cross-checking, and preterm follow-up workflows.
For very preterm infants, ROP screening is commonly anchored to postmenstrual age and postnatal age. The date shown is a calculation aid only; confirm eligibility and timing with local ROP criteria.