Should parity be included in customised fetal weight standards for identifying small-for-gestational-age babies?...

Parity is one of several parameters used to customise fetal growth norms. However, it is uncertain whether the lower birthweight of babies born to primiparous women reflects physiological or pathological variation. Our aim was to assess the impact of adjusting for parity in identification of small-for-gestational-age (SGA) births.

Comparison of two customised definitions of SGA with and without parity.

Routinely collected data in five tertiary maternity hospitals in France.

A total of 51 126 singleton births without malformations from 1997 to 2002.

Characteristics of mothers and babies and adverse pregnancy outcomes for SGA and non-SGA births were compared using customised definitions with and without parity.

Neonatal morbidity and mortality.

SGA births among primiparas increased from 14.9 to 18.0% when parity was excluded. Overall rates of SGA rose from 14.4 to 15.0%. Newly defined cases of SGA were babies of primiparas. They had higher rates of admission to a neonatal unit and caesarean section than babies reclassified as non-SGA. Perinatal mortality was 9.1 per thousand (parity included) and 9.7 per thousand (parity excluded) and did not differ significantly from babies classified as non-SGA by both standards (5.4 per thousand).

Adjustment for parity markedly decreased the proportion of primiparas diagnosed with SGA babies but did not appear to improve the identification of high-risk babies. Removing parity would simplify the customised definition of SGA and would eliminate the need for the assumption that lower birthweight for primiparous women is normal.