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“Perceptions of fetal size influence interventions in pregnancy, BU study finds,” reads the EurekAlert article title. This is hardly breaking news, as every other study of suspected macrosomia has found the same thing; however, this latest study confirms the role of clinician bias.
Using data from Listening to Mothers III, a survey of U.S. women giving birth in 2011-2012, investigators looked at the subgroup of 1960 women with no prior cesarean and compared the 566 participants who responded “yes” to the question “Did your maternity care provider tell you that your baby might be getting quite large?” with the rest. While 1/3 of the women were told that their babies might be big, only 10% delivered a baby weighing 4000 g (8 lb 13 oz) or more, the usual definition for macrosomia. Among women given this news, only 20% had babies in the macrosomic weight range.
Predictably, suspected big baby resulted in more inductions (70% vs. 51%), both medical (60% vs. 40%) and self-induction attempts (43% vs. 25%). Not so predictably, it didn’t increase cesarean rate (22% vs. 18%), although it did increase the number of women wanting (33% vs. 7%) and having (13% vs. 6%) a scheduled cesarean.
Investigators found that race (35-38% women of color vs. 27% white women), payment source (self-pay 46%, private 34%, public 27%), BMI (42% BMI > 30 vs. 28% BMI 19-25), and gestational diabetes (GD) status (65% yes vs. 15% no) strongly influenced whether women would be told their baby would be big; however, the actual percentage with infants weighing 4000 g or more failed abysmally to justify the prediction: 8-10% women of color vs. 11% white, 6% self-pay vs. 10% private vs. 11% public. Even with factors associated with higher birthweight, the true association was much weaker than clinicians thought: 15% BMI > 30 vs. 8% BMI 19-25; 15% GD vs.9% no GD. In other words, women were diagnosed and treated according to unfounded clinician beliefs.
Women also, point out the study authors, weren’t treated according to the evidence. They noted that studies consistently find fetal weight estimates to be unreliable enough to be clinically useless, and ACOG guidelines reject suspected macrosomia as an indication for induction or planned cesarean except when fetal weight estimates predict birthweight greater than 5000 g (11 lbs 1 oz.).
If given this news, cover bets by seeking information from knowledgeable sources on how to best to promote safe, physiologic birth with a bigger baby. None of the tips will do any harm if the baby isn’t super-size after all.