Length of gestation
Another variable which must be considered in this connection
is the length of pregnancy. It is possible that
low-birth-weight infants of smoking mothers are “more
mature” than are low-birth-weight infants of nonsmoking
mothers and this may explain their lower mortality.
Infants weighing 2500 grams or less of smoking mothers
have been in utero, on the average, three days longer than
the low-birth-weight infants of nonsmoking mothers
(253.3 days compared to 250.4 days). It is therefore
necessary to explore this question further and investigate
whether this is a reasonable explanation for their favorable
mortality.
Figure 5 shows that the distribution by weeks of gestation
was approximately the same for infants of smoking
and non-smoking mothers for both whites and blacks.
Apparently, therefore, there has not been a displacement as
far as length of pregnancy is concerned. Moreover, the
mean birth weight of infants of smoking mothers was
lower than that of nonsmoking mothers over the entire
span of the gestational age distribution (table 6). Infants of
smoking mothers weighed approximately 200 grams less
than those of nonsmoking mothers. The difference was of
about the same order of magnitude in nearly each week of
gestation.