Gestational age assessment and preterm classification — formative SAQs

SAQ 1 (10)

A newborn weighs 2100 g. The obstetric record shows a first-trimester crown-rump-length ultrasound consistent with a last menstrual period of 39 weeks and 4 days. The mother had irregular cycles. ^[3]

1. Explain why this infant is not preterm, and state the correct classification of size-for-gestational-age. (2) ^{[2] [7]}
2. Describe how you would confirm the gestational age postnatally, and the limitations of that method in this infant. (3) ^[1]
3. Outline the dating hierarchy and the reconciliation rule you would apply if the last menstrual period and the first-trimester ultrasound disagreed by 10 days. (3) ^{[3] [4]}
4. State the term sub-category this infant falls into and why the category matters. (2) ^[5]

Model answer

Classification, not prematurity. Gestational age is time from the first day of the last menstrual period to the day of birth, not weight. This infant is term (39+4). Because the weight is below the 10th centile for gestational age, the correct classification is small-for-gestational-age term infant, not preterm. Treating the low weight as prematurity would misdirect screening and follow-up. ^{[2] [7]}

Postnatal confirmation and its limits. Perform the New Ballard Score, combining six neuromuscular and six physical criteria, which is reliable to about plus or minus two weeks. In a growth-restricted or oedematous infant the physical criteria (skin, breast bud, plantar creases) can overstate maturity, and the score's accuracy falls, so antenatal ultrasound dating should anchor the estimate and the score is confirmatory. ^[1]

Dating hierarchy and reconciliation. First-trimester crown-rump-length ultrasound is the reference standard at about 5 to 7 days. When it disagrees with the last menstrual period by more than about 7 days in the first trimester, the ultrasound is preferred, especially with irregular cycles. Here the two agree, so the 39+4 estimate stands. ^{[3] [4]}

Term sub-category. At 39+4 the infant is full term (39+0 to 40+6), which carries the lowest morbidity within term; this is why the Defining Term Pregnancy Workgroup split the old single category. ^[5]