"Big Baby" Diagnosis Increases Chance of C/S Without Improving Newborn Outcomes

Prenatal Diagnosis of Suspected Fetal Macrosomia Increases Risks of Cesarean Section and Maternal Morbidity without Improving Newborn Outcomes

Sadeh-Mestechkin, D., Walfisch, A., Shachar, R., Shoham-Vardi, I., Vardi, H., & Hallak, M. (2008). Suspected macrosomia? Better not tell. Archives of Gynecology and Obstetrics, doi: 10.1007/s00404-008-0566-y. [Abstract]

Summary: In this prospective observational trial, researchers followed 145 women thought to be carrying babies weighing more than 4000g (about 8lb, 13oz) to evaluate the reliability of sonographic and clinical estimates of fetal weight and to determine whether a diagnosis of "suspected macrosomia" affects pregnancy management or outcomes. To answer these questions, they first divided the "suspected macrosomia" into two subgroups depending on whether the babies in fact weighed more or less than 4000g. This resulted in a "false-positive" subgroup of 82 babies thought to be macrosomic but actually weighing less than 4000g and a "true-positive" group of 63 babies thought to be macrosomic and indeed weighing more than 4000g.

The researchers compared outcomes between these two groups, then compared the "true-positive" group to all other births of macrosomic infants to women admitted to the same hospital in the study period (i.e., women giving birth to infants weighing over 4000g but who did not have a prenatal diagnosis of suspected macrosomia).

The study confirms an existing body of literature that tells us that prenatal methods for detecting macrosomia are not reliable—only 44% of babies suspected to weigh over 4000g actually did. Clinical estimates (those performed by a care provider using palpation) were more reliable than ultrasound estimates, but were still off by more than 10% in one out of every six cases.

The 145 women with suspected macrosomia were assigned by the admitting doctor to elective cesarean surgery (16%) induction of labor (39%) or observation awaiting labor (46%). Thirty-five of the 56 women who underwent induction and 47 of the 66 women who were admitted in spontaneous labor gave birth vaginally, for an overall vaginal birth rate of 57% (including 2% who had instrumental vaginal births). The remaining women gave birth by cesarean surgery either before (21%) or during (22%) labor.

When researchers compared outcomes of the "true-positives" with the "false-positives" within the "suspected macrosomia" group, there were no differences in maternal or infant complications.

When the researchers compared the true-positive macrosomic infants in the study group (those suspected to weigh and actually weighing >4000g) to the macrosomic infants in the comparison group (those not suspected to be macrosomic prior to birth), they found much higher rates of intervention and related morbidity in the study group. The cesarean surgery rate for macrosomic infants in the suspected macrosomia group was 57%, compared with only 17% for macrosomic infants in the comparison group (absolute difference 40%). Likewise, 25% of macrosomic infants in the study group were induced compared with 14% in the comparison group (absolute difference = 11%). There was no difference in shoulder dystocia between the two groups. However, maternal morbidity (including hemorrhage, wound infection, wound dehiscence, fever, and use of antibiotics) was significantly higher in study group, most likely because of the much higher rate of cesarean surgery in this group.

Significance for Normal Birth: The concern that a baby is growing "too big" is one of the most common reasons cited for induction of labor and also prompts decisions to perform cesarean surgery both before and during labor (Declercq, Sakala, Corry, & Applebaum, 2006). However, there is strong and consistent evidence that elective induction or cesarean surgery for "suspected macrosomia" does not improve outcomes and expert bodies including the American College of Obstetricians and Gynecologists oppose routinely interfering when a baby is suspected to be large (American College of Obstetricians & Gynecologists, 2004).

This study, summed up by its title, "Suspected macrosomia? Better not tell" provides damning evidence that shows clearly that the provider's belief that the baby is "too big" is itself a strong risk factor for injudicious intervention and poor health outcomes.

An initial suspicion that the baby is large may instill fear in the pregnant woman which may impede both her confidence in her body and her labor progress. Slow labor progress reinforces the suspicion that the baby is big and more aggressive management ensues. This management often hinders the woman's ability to move freely and assume the positions that may help her baby negotiate through her pelvis, further slowing progress and reinforcing the perceived need for surgical intervention. Based on this study and previous evidence, women should strongly consider refusing tests late in pregnancy intended to estimate fetal weight.

The estimate itself may be bad for her health because the care provider's expectation that the baby will be macrosomic appears to increase both unnecessary medical intervention and the morbidity that may accompany it.

References: American College of Obstetricians and Gynecologists. (2004). Ultrasonography in pregnancy. ACOG Practice Bulletin No. 58. Obstetrics and Gynecology, 104, 1449–58.
Declercq, E., Sakala, C., Corry, M. P., & Applebaum, S. (2006). Listening to mothers II: Report of the second national U.S. Survey of women's childbearing experiences. New York: Childbirth Connection.), 116-122.