While some doctors consider inducing labor to be pretty routine, it can be a big decision for an expecting mother. An induction can squash hopes of having a natural birth, increase risk of an unplanned Cesarean birth, and increase risk of infection. In addition, induction before 39 weeks (while it may sometimes be medically necessary) can negatively impact a baby’s development. As with any medical procedure, it’s important to carefully consider whether the benefits outweigh the risks. According to the U. S. Listening to Mothers III Survey on Pregnancy and Birth, 16% of inductions were due to concerns over baby’s size [1]. With 4 in 25 inductions of labor occurring due to the baby’s size, this poses two important questions: 1) How do we know how big a baby will be before he or she is born? And 2) is it important to induce labor early before a larger-than-average baby grows any bigger?
Can we actually know ahead of time whether a baby will be big?
A friend of mine was told at 39 weeks gestation that her baby was nearly nine pounds, and that if she wanted to try for a vaginal birth they would need to induce labor. After her failed induction ended in a C-section the next day, she was handed her healthy, seven and a half pound baby. How did this happen?
Ultrasound weight predictions in third trimester are usually inaccurate
At my friend’s 39-week appointment, her medical provider used ultrasound to take measurements of her baby, and the machine calculated a weight estimate based on these measurements. While ultrasound measurements are quite accurate early on, especially during the first trimester, measurements done during the third trimester are far less accurate. A sonographer I work with explained it to me like this: early on in pregnancy, when the little embryo or fetus is much smaller than the ultrasound probe, it is easy to get the entire embryo or fetus in view for measurements; a smaller, more focused portion of the ultrasound array is being used to get these measurements. Later on in pregnancy, the fetus is much larger than the probe. Only part of the fetus can be seen at one time, and a wider portion of the ultrasound array needs to be used to take measurements, increasing the error range of these measurements.
In the third trimester, measurements can vary by a week and a half in either direction, giving a total measurement range of three weeks (ultrasound machines will calculate gestational age based on fetal measurements, hence measuring the error range in number of weeks). So if your baby is measuring larger or smaller than expected during an ultrasound exam, this may be simply due to the inherent inaccuracy of third trimester ultrasound measurements. In fact, a 2008 study on macrosomia (aka, “large body” or a larger-than-average baby, often considered to be a baby bigger than 8 lbs 13 oz, or sometimes bigger than 9 lbs, 15 oz., depending on the researcher/study) found that ultrasound measurements only accurately predicted a large baby 43.5% of the time [2].
However, while ultrasound exams cannot accurately predict a baby’s weight, there may be other reasons to suspect a larger-than-average baby, such as when the pregnant mother has diabetes or gestational diabetes. In these cases, there may be other factors for you and your doctor to consider besides size when deciding whether to have an induction.
Is there any benefit to inducing labor early, before a larger-than-average baby grows any bigger?
Maybe the real question here is, “will my baby get stuck if he/she gets any bigger?” When a baby’s shoulders get stuck during delivery this is called shoulder dystocia. However, while this could potentially lead to injury for both the mother and baby, shoulder dystocia is usually resolved safely with a little extra maneuvering from the care provider. According to a 2019 study by Beta et al., the rate of shoulder dystocia was 0.6% for babies under 8 lbs., 12 oz.; 6% for babies between 8 lbs. 13 oz. and 9 lbs., 14 oz.; and 14% for babies 9 lbs., 15 oz. and up [3]. In a 2013 literature review by Rebecca Dekker, PhD, RN and Anna Bertone, MPH, the authors determined that the rate for permanent nerve injury in babies due to shoulder dystocia was relatively rare even for the largest weight group (9 lbs. 15 oz. and up) with a rate of 1 in 175 births, or 0.005% [4].
The official stance of the American College of Obstetricians and Gynecologists (ACOG) since 2016 is that suspected fetal macrosomia is not a valid reason for early induction of labor (before 39 weeks). While the research studies they examined had relatively small sample sizes, results suggested that inducing labor before 39 weeks more than doubles the risk of C-section without decreasing the risk of shoulder dystocia. As with virtually every other area of obstetric management, further research is needed.
Vaginal tear rates with big babies
On the subject of whether or not larger babies will “fit” through the birth canal, the risk to mom’s perineum with larger babies is also less than you might expect. The 2019 study by Beta et al. (mentioned above) also found that the risk of severe tear for average size babies (8 lbs., 12 oz. and less) was 0.9%, 1.7% for “big babies” (8 lbs., 13 oz. to 9 lbs., 14 oz.), and 3% for even larger babies (9 lbs., 15 oz. and up) [3]. In contrast, the use of forceps during delivery carries nearly four times higher risk of perineal tearing compared to large birth weight [5]. Luckily, there are also options like perineal massage that may help prevent perineal tearing!
Induction for big babies: The power of perception
In a 2008 study by Sadeh-Mestechkin et al., researchers found that whether or not the medical provider thought the baby would be “too big” made more of a difference than the actual size of the baby [2]. This study compared the outcomes of births where the babies were suspected to be large beforehand (and did end up being large), with births where the babies that had large birth weights were not classified as large beforehand. All of the births involved in the comparison were babies weighing 8 lbs., 13 oz. and up, but births where the baby’s size was estimated beforehand had significantly higher induction and C-section rates than babies whose large birth weights were a surprise.
So what does this tell us? While having a baby with a larger birth weight is unlikely to cause complications during labor, concerns over having a baby with a larger birth weight do increase the rate of C-sections. This is likely due to one of two reasons (or both). Either a patient elects to have a C-section because of the perceived risk of having a vaginal delivery with a larger baby, and/or the attending physician is less willing to accept normal bumps in the road during labor and recommends a C-section sooner than he or she might otherwise do with babies not diagnosed with macrosomia.
While labor induction and C-sections can be life-saving options in complex birth or emergency situations, starting life a little larger than normal qualifies as neither. What’s more, the only way to know for sure how big a baby will be at birth is to wait for him or her to be born!
References:
[1] Declercq ER, Sakala C, Corry MP, Applebaum S, Herrlich A. “Listening to MothersSM III: Pregnancy and Birth.” New York: Childbirth Connection, May 2013. [2] Sadeh-Mestechkin D, Walfisch A, Shachar R, Shoham-Vardi I, Vardi H, Hallak M. “Suspected macrosomia? Better not tell.” Arch Gynecol Obstet. vol. 278, no. 3 (2008): pp. 225-30. doi: 10.1007/s00404-008-0566-y. Epub 2008 Feb 26. PMID: 18299867. [3] Beta J, Khan N, Khalil A, Fiolna M, Ramadan G, Akolekar R. “Maternal and neonatal complications of fetal macrosomia: systematic review and meta-analysis.” Ultrasound Obstet Gynecol. vol. 54, no. 3 (2019):pp. 308-18. doi: 10.1002/uog.20279. PMID: 30938004. [4] Dekker, R., & Wilson, E. (2021, October 20). “What is the evidence for induction or C-section for a big baby?” Evidence Based Birth®. https://evidencebasedbirth.com/evidence-for-induction-or-c-section-for-big-baby/ [5] Smith LA, Price N, Simonite V, Burns EE. “Incidence of and risk factors for perineal trauma: a prospective observational study.” BMC Pregnancy Childbirth. vol 13, no. 59 (2013). doi: 10.1186/1471-2393-13-59. PMID: 23497085; PMCID: PMC3599825.Additional Reading:
The Fourth Trimester Guidebook: Postpartum Nutrition
The Fourth Trimester Guidebook: Postpartum Healing and Exercise
The Fourth Trimester Guidebook: Your Postpartum Mental Health Matters
So You’re Going to Give Birth in the United States…What Are Your Options?
