How do you calculate postconception age in a preterm infant?

How to Calculate Postconception Age in Preterm Infants

Postconceptional age is calculated by adding the gestational age at birth to the chronological age (postnatal age) since birth. 1, 2

The Basic Formula

• Postconceptional Age = Gestational Age at Birth + Chronological Age (weeks since birth) 1, 2
• For example, an infant born at 28 weeks gestation who is now 8 weeks old has a postconceptional age of 36 weeks 1
• This calculation assumes accurate determination of gestational age at birth, which is optimally established by first-trimester ultrasound with crown-rump length measurement between 8-13 weeks (accurate within ±5-7 days) 3

Clinical Applications and Why This Matters

Postconceptional age serves as a critical clinical marker for predicting hospital discharge timing, assessing developmental milestones, and defining disease severity in preterm infants. 1, 2

For Hospital Discharge Planning

• Most preterm, low-birth-weight infants meet discharge criteria at a postconceptional age of 35 to 37 weeks 2
• Infants with birth weights 1750-2240g typically discharge at a mean postconceptional age of 36.0 ± 1.4 weeks 2
• For extremely low birth weight infants (<1000g), discharge at 37 to 42 weeks postconceptional age is more realistic 2

For Defining Bronchopulmonary Dysplasia (BPD) Severity

• The definition of BPD uses postconceptional age as a key criterion: oxygen requirement at 36 weeks postconceptional age (rather than 28 days postnatal age) provides more accurate prediction of subsequent respiratory morbidity in very preterm infants 1
• This is particularly important for infants born at ≤30 weeks gestation, where postconceptional age-based definitions better reflect pulmonary outcomes 1

For Developmental Assessment

• Behavioral and electrophysiological assessments should account for postconceptional age rather than chronological age alone, as gestational age at birth significantly affects neurodevelopmental function even when corrected to equivalent postconceptional ages 4
• Fullterm infants show significantly better behavioral function than preterm infants studied at the same postconceptional age of 42 weeks, indicating persistent effects of prematurity 4

Important Caveats in Gestational Age Determination

The accuracy of postconceptional age calculation depends entirely on accurate initial gestational age assessment. 1, 3

• Ultrasound performed at <24 weeks estimates gestational age within 5-14 days 1
• When discrepancy ≥5 days exists between last menstrual period (LMP) and crown-rump length dating, the ultrasound-based gestational age should be adopted 3
• For patients with irregular menstrual cycles, standard LMP-based calculations require adjustment by adding the difference between the patient's cycle length and the standard 28-day cycle 3
• Inaccurate gestational age assignment introduces uncertainty into all postconceptional age-dependent predictions and clinical decisions 1

Periviable Period Context

• The periviable period is defined as delivery from 20 0/7 to 25 6/7 weeks gestation 1
• Outcomes at these extremely early gestational ages vary dramatically: survival rates are 5-6% before 23 weeks, 23-27% at 23 weeks, 42-59% at 24 weeks, and 67-76% at 25 weeks 1
• Postconceptional age becomes particularly relevant for these survivors, as their prolonged NICU courses require ongoing assessment referenced to expected developmental milestones 1