National Women's Annual Clinical Report 2010

of women under 20 with SGA babies smoked in pregnancy compared to 27% of women
under 20 with non SGA babies.
With regard to the apparent higher rate of SGA babies and lower rate of LGA babies
observed in Asian women in this report, we are currently updating the New Zealand
customised birthweight centile coefficients with data from a larger cohort of births. The
coefficient for Chinese ethnicity has reduced since the last version of the calculator which
is likely to reduce the proportion of Asian SGA babies and increase the proportion of
Asian LGA in future reports (Anderson et al personal communication).
Table 17: Rates of SGA and LGA as defined by customised birthweight centiles (compared
to AGA) by demographic characteristics continued (n=babies)
Total
Babies
Customised
birthweight
90 th % (LGA)
N n % n % n %
Total
Smoking at
booking
7866 910 11.6 6216 79.0 740 9.4
Currently
615 129 21.0 445 72.4 41 6.7
smoking
No or not
smoking in last
month
7203 774 10.7 5734 79.6 695 9.6
Unknown
BMI
48 7 14.6 37 77.1 4 8.3
35 552 70 12.7 407 73.7 75 13.6
Missing data
Plurality
222 35 15.8 162 73.0 25 11.3
Singleton 7556 802 10.6 6019 79.7 735 9.7
Multiple 310 108 34.8 197 63.5 5 1.6
The increased risk of SGA among over-weight and obese women is likely explained by
the higher rate of hypertensive disease among these women (approximately 15% have
chronic or pregnancy induced hypertension compared with 6% of women with normal
BMI). Customised centiles are designed to be used in singleton pregnancies so the
finding that 34.8 % of infants from multiple pregnancies are SGA needs to be interpreted
with some caution although it is well recognised that SGA is more common in multiple
pregnancies.
Consistent with international literature women who smoke have an elevated (two-fold)
risk of SGA infants. Further exploration of independent risk factors for SGA infants will
require multivariate analysis and is planned as part of ongoing research.
The only group who appear to have an increased risk of LGA infants are women with BMI
>35. Maternal obesity is a known risk factor for LGA babies and the associated increased
rate of gestational and type 2 diabetes will also contribute to this increased risk.
49