The distribution of maternal characteristics in relation to
birthweight, placental weight, and placental ratio is shown
in Table 1. Birthweight of infants in the placental abruption
group was shifted toward lower weights, whereas
weights of the placenta were similar between the abruption
and nonabruption groups (Figure 1). The proportion
of infants that weighed <10th centile based on birthweight
was higher among abruption than nonabruption
births (Table 2). However, placental weight <10th centile
was not associated with abruption, overall. Placental ratio
<10th centile was lower among abruption than nonabruption
births.
Gestational age-specific mean birthweight and placental
weights were, on average, considerably lower among
abruption than nonabruption births, but this difference
was only evident among preterm births (Figure 2). In
contrast, mean placental ratios were higher among abruption
births at early gestations, and began to steeply
decline as gestation advanced. We examined risks of placental
ratio <10th centile in relation to abruption (Figure
3). The frequency of placental ratio <10th centile was
lower among abruption than among nonabruption births
for preterm gestations (risk ratio 0.4, 95% CI 0.2–0.8),
but not at term (risk ratio 0.9, 95% CI 0.6–1.2).
We examined the effect on the overall findings of
excluding stillbirths. When we included the 1009 stillbirths
in the analytical cohort, the results were similar to
those presented above (data not shown). Finally, exclusion
of births with extreme birthweight-for-gestational
age (i.e. <3rd and >97th centiles) also did not have any
effect on the findings
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