Are babies born to short, primiparous, or thin mothers "normally" or "abnormally" small?

To examine whether the smaller size of infants born to primiparous, short, or thin mothers is associated with increased risks of perinatal mortality.

We compared gestational age-specific patterns of "revealed" small-for-gestational-age (SGA) birth (number of SGA births expressed as a proportion of fetuses remaining in utero at each gestational age) with the patterns for perinatal mortality among singleton late fetal deaths and live births (n = 791,523) to Swedish mothers in 1992 to 2001.

Based on a single standard for SGA, primiparae were at substantially higher risk of revealed SGA throughout gestation, paralleling the pattern for perinatal mortality. However, for short and thin women, risks of revealed SGA were much more consistent with those for perinatal mortality when SGA was based on height-specific or body mass index-specific standards, respectively, rather than on the single standard. Overweight and obese mothers had lower revealed SGA rates based on either standard but higher perinatal mortality rates.

Slower fetal growth due to maternal short stature or low prepregnancy body mass index appears to be physiologic, whereas the slower growth of fetuses born to primiparous women is associated with higher risks of perinatal death.