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]

SAQ 2 (10)

A 35-week infant is admitted for feeding immaturity and jaundice. The parents ask why their baby, who "looks almost full term", cannot go home yet. [6]

1. Classify this infant by gestational age and explain why the band is not benign. (3) [6]
2. Outline the discharge-readiness criteria that must be met before discharge. (3) [6]
3. Describe how you would plot this infant's growth and interpret milestones, and for how long. (2) [7] [8]
4. State which screening is gestational-age-triggered at this band and which is not required. (2) [1]

Model answer

Classification and why it matters. This is a late-preterm infant (34+0 to 36+6). Late-preterm infants are the largest preterm group by number and, despite looking almost mature, carry morbidity and readmission rates substantially higher than term, driven by feeding immaturity, temperature instability, hypoglycaemia and jaundice. [6]

Discharge-readiness criteria. Thermal stability in an open cot, full oral feeds with sustained weight gain, no significant apnoea for 5 to 7 days, stable weight trajectory, and immunisation on chronological age, with parental education and a follow-up plan in place. [6]

Growth and milestones with correction. Plot on Fenton charts to 50 weeks postmenstrual age, then transition to WHO or INTERGROWTH-21st standards. Correct milestones for the 5 weeks born early, and continue correcting until about 2 years, after which most children align with chronological peers. [7] [8]

Screening at this band. A hearing screen before discharge and the newborn bloodspot are routine. Routine cranial ultrasound and retinopathy of prematurity screening are not triggered at 35 weeks in an infant over 1500 g, because those screens are gated by gestational age below 32 weeks (cranial) or below 30 to 32 weeks or 1500 g (retinopathy). [1]