How to Calculate Postnatal Age in a Neonate
Postnatal age is simply the number of days or weeks that have elapsed since birth, starting from day 0 at the moment of delivery. This is a straightforward chronological calculation that does not require adjustment for gestational age at birth.
Basic Calculation Method
- Postnatal age = Current date - Date of birth 1
- Express the result in days for the first month, then transition to weeks for ongoing assessments 1
- Day 0 begins at the moment of delivery and continues through the first 24 hours of life 1
Corrected Postnatal Age for Preterm Infants
When assessing preterm infants for developmental milestones, growth parameters, or research purposes, you may need to calculate corrected postnatal age (also called postconceptional age):
- Corrected postnatal age (in weeks) = Gestational age at birth + Chronological postnatal age 1
- This calculation assumes term is 40 weeks gestational age 1
- For example: An infant born at 28 weeks gestation who is now 8 weeks old has a corrected postnatal age of 36 weeks 2
Clinical Applications
For neonatal mortality reporting:
- Neonatal mortality is defined as death within 28 days of birth, using uncorrected postnatal age 1
- Early neonatal mortality occurs in the first 7 days after birth 1
- Late neonatal mortality occurs between 8 and 28 days after birth 1
For fluid and electrolyte management:
- The immediate postnatal phase (Phase I) typically lasts hours to days and is characterized by physiological adaptation 1
- Phase II extends from maximum weight loss until return to birth weight, usually by 7-10 days of life in term infants 1
- Phase III represents stable growth with continuous weight gain 1
For preterm infant discharge planning:
- Most preterm, low-birth-weight infants meet discharge criteria at a corrected postnatal age (postconceptional age) of 35-37 weeks 2
- Very low birth weight infants (<1000 g) typically discharge at a corrected age of 37-42 weeks 2
Common Pitfalls to Avoid
- Do not confuse postnatal age with gestational age - postnatal age starts at birth, while gestational age reflects fetal development from conception 1, 3, 4
- Do not use corrected age for acute clinical decisions - use actual chronological postnatal age for medication dosing, fluid management, and acute care protocols 1
- Reserve corrected postnatal age for developmental assessments - this adjustment is primarily useful for evaluating growth, neurodevelopment, and expected physiological milestones in preterm infants 1, 5, 2