Temporal changes in small-for-gestational age live births associated with obstetric intervention in the United States
Iatrogenic early delivery through labour induction/cesarean delivery given suspected fetal/maternal compromise is the foundation of modern obstetrics. It has been hypothesized that increasing rates of obstetric intervention may be responsible for decreasing rates of small for gestational age (SGA) births at the population level, as fetuses that would have been delivered spontaneously at later gestations after a period of impaired growth are instead being delivered earlier, prior to the onset of SGA.
METHODS:
Population-based data on singleton live births born between 24-43 weeks of gestation in the United States from 1990 to 2010 were obtained. The fetuses-at-risk approach was used to calculate the gestational-age specific rates of SGA and obstetrical intervention. Kitagawa decomposition was used to assess the relative contribution of changes in the gestational age distribution and the gestational age-specific SGA rates to the overall temporal changes in SGA rates.
RESULTS:
The rate of SGA births declined steadily from 10.1% in 1990-1992 to 8.9% in 2002-2004; however, starting in 2005-2007, the SGA rate slowly increased and was 9.1% in 2008-2010. The changing rates of early delivery associated with obstetric intervention mirrored this change, with a 45.8% increase in labour induction/cesarean delivery between 1990-1992 to 2002-2004, followed by a 29.8% decrease between 2002-2004 and 2008-2010. The Kitagawa decomposition indicated that the initial decline in SGA rates was entirely due to changes in the gestational age distribution, whereas the increase in the later time period was due to changes in the gestational-age specific SGA rates. This change in the gestational-age specific SGA rates was only observed in the term/post-term population.
CONCLUSIONS:
Changing rates of obstetric intervention are associated with temporal patterns in SGA rates. Further study is necessary to elucidate the appropriate balance between early delivery at appropriate birth weight for gestational age or later delivery of SGA infants.