Gestational age assessment and preterm classification

Overview & Definition

Picture the delivery room at two in the morning. A baby is born weighing 2100 g, and the question that will shape every decision for the next two years is deceptively simple: how old, in weeks, is this baby? The answer is not the weight, and it is not always obvious from the appearance — it is the gestational age, the completed weeks from the first day of the mother's last menstrual period to the day of birth. [6]

Gestational age is the organising principle of neonatal medicine because it predicts, better than any other single variable, which organ systems are immature enough to fail. A 26-week infant and a 34-week infant are different patients — different lung, brain, gut and temperature risk — and the plan flows from the band they fall into. Establishing that number accurately, and reconciling antenatal with postnatal estimates when they disagree, is one of the first high-stakes tasks the paediatrician owns. [6] [1]

Three related ages travel with gestational age and the candidate must keep them separate. Postmenstrual age is the gestational age at birth plus the chronological age, used in the first weeks to time screening such as the first retinopathy examination around 31 to 34 weeks postmenstrual age. Chronological age is the time since birth. Corrected age is the chronological age minus the number of weeks the baby was born early, and it is the age you use to plot growth and interpret milestones for the first two years. [18] [19]

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