SAN FRANCISCO – The risk of stillbirth in pregnancies complicated by a fetus that is small for gestational age rises after the pregnancy reaches term, supporting prompt delivery, according to a retrospective cohort study.
Investigators led by Dr. Amanda S. Trudell, an obstetrician at Washington University in St. Louis, studied more than 3,000 women with singleton gestations complicated by small for gestational age (SGA) but otherwise healthy fetuses.
Analyses showed that the cumulative risk of stillbirth began to rise after gestation exceeded 37 weeks. It was significantly elevated by nearly threefold for women delivering at 39 weeks and by nearly sevenfold for those delivering at 40 or more weeks.
The number of deliveries needed to prevent a single stillbirth (number needed to treat) was 204 at 39 weeks and 60 at 40 or more weeks.
"Our data demonstrates a continuous rise in the cumulative risk of stillbirth after 37 to 38 weeks," Dr. Trudell reported when presenting the data at the Pregnancy Meeting, the annual meeting of the Society for Maternal-Fetal Medicine.
"Without a randomized controlled trial to evaluate stillbirth, and weighing the gravity of stillbirth against the morbidities of early-term delivery, given the favorable number needed to treat to prevent one stillbirth, we advocate delivery of SGA pregnancies in the 37th week and not beyond the 38th week," she said.
Session attendee Dr. Harvey Kliman of Yale University in New Haven, Conn., asked, "Do you have any data on placentas of these cases, because I think it would be important to understand why they are SGA and why they are IUFD [intrauterine fetal demise]."
Such data are not available for this study cohort, Dr. Trudell replied.
"I would just make a pitch to everybody: Don’t throw out your best defense. I do a lot of medicolegal work and will tell you, it’s important to have it," Dr. Kliman advised. "And also for these studies, we need to understand the mechanism behind this to really fix this problem."
Another attendee wondered about the definition of SGA used in the study, which was a birth weight of less than the 10th percentile.
"Have you considered using things like customized growth curves? Some people think there are better ways to truly define who is at risk and who is not at risk," he commented.
The investigators did not look at customized growth curves in this study, according to Dr. Trudell; however, they did look at a more stringent threshold for SGA, set at the 5th percentile. In that analysis, the cumulative risk of stillbirth became significantly elevated at 38 weeks, with a 2.3-fold higher risk than at 37 weeks.
"The delivery timing of SGA pregnancies weighs the competing risks of neonatal morbidity associated with early-term delivery versus the risk of stillbirth associated with expectant management," she said, giving some background to the research.
In 2010, the DIGITAT (Induction Versus Expectant Monitoring for Intrauterine Growth Restriction at Term) trial demonstrated no increase in maternal or neonatal risk with induction of SGA fetuses at 37 weeks, compared with expectant monitoring, she noted (BMJ 2010;341:c7087). "The authors concluded that either induction or expectant monitoring were acceptable management strategies for presumed growth restriction at term. Due to the rarity of stillbirth, the trial was underpowered to evaluate this outcome," Dr. Trudell said.
Starting with Washington University’s prospective perinatal database, Dr. Trudell and her colleagues identified 3,333 women with a singleton gestation of at least 37 weeks that had an SGA fetus but was not complicated by major anomalies or aneuploidy.
Overall, 0.6% of the women had a stillbirth, according to data reported at the meeting.
Life table analyses showed that the cumulative risk of stillbirth (which reflects risk through time) was 28, 41, 77, and 194 per 10,000 women at 37, 38, 39, and 40 or more weeks, respectively.
The corresponding conditional risk of stillbirth (which captures risk at just a single point in time and is conditional on survival to that point in time) was 28, 13, 36, and 120 per 10,000 women.
"The cumulative risk of stillbirth is the preferred method when attempting to answer our clinical question of delivery timing. ... Notice that the conditional risk uniformly underestimates the stillbirth risk," Dr. Trudell noted.
Compared with women who delivered at 37 weeks, those delivering at 38 weeks had a nonsignificant 1.5-fold higher risk of stillbirth, and those delivering at 39 weeks and at 40 or more weeks had respective significant 2.8- and 6.9-fold higher risks.