Objectives To determine the following: (1) the proportion of babies reclassified as small or appropriately grown using customised and population centiles; and (2) the relative risks of perinatal morbidity, including abnormal umbilical Doppler studies, in babies classified as small for gestational age (SGA) and appropriate for gestational age (non-SGA) using the two centile calculations.
Design Cohort study in SGA and general hospital populations.
Setting National Women's Hospital, Auckland, NZ.
Population A cohort of SGA pregnancies (n= 374) and a general obstetric population (n= 12,879).
Methods Pregnancy outcomes were compared between ‘non-SGA both’ (≥10th% by population and customised centiles) and those who were ‘SGA both’ (<10th% by population and customised centiles), ‘SGA customised only’ (SGA by customised but non-SGA by population centiles) and ‘SGA population only’ (SGA by population but non-SGA by customised centiles).
Main outcome measures Maternal and newborn morbidity and perinatal death.
Results In the SGA cohort 271 (72%) babies were ‘SGA both’, 27 (7%) were ‘SGA customised only’, 32 (9%) were ‘population SGA only’ and 44 (12%) were ‘non-SGA both’. In the general obstetric population 863 (6.7%) babies were ‘SGA both’, 445 (3.5%) were ‘customised SGA only’, 285 (2.2%) were ‘population SGA only’ and 11,286 (88%) were ‘non-SGA both’. Perinatal death and newborn morbidity including nursery admission and long hospital stay were increased and comparable between ‘SGA both’ and ‘customised SGA only’ in both study populations. Newborn morbidity was low and comparable between ‘population SGA only’ and ‘non-SGA both’. No perinatal deaths occurred in ‘population SGA only’ babies. Abnormal Doppler studies were more common in ‘SGA both’ or ‘customised SGA only’ but not in ‘population SGA only’ groups compared with ‘non-SGA both’.
Conclusions Customised birthweight centiles identified small babies at risk of morbidity and mortality. Use of customised centiles is likely to detect more babies at risk of perinatal morbidity and mortality than would be detected by population centiles.