Have Cesareans Driven Evolution Toward Bigger Babies?


“The great tragedy of Science: the slaying of a beautiful hypothesis by an ugly fact.”
Thomas Huxley

Reverberating through the internet is an article by a group of scientists theorizing that cesarean availability has altered the course of human evolution, resulting in an epidemic of babies with heads too big to pass through their mother’s pelvises. The article presents a complex argument with formulas, calculations, and graphs showing the tension between maximizing fetal size and the limits of a pelvis designed for walking upright on two legs, a balance, they conclude, disrupted by the option of surgical delivery. It’s all very impressive, and it’s hogwash.

For one thing, the underlying assumptions are false. The authors assume that the dimensions of the mother’s pelvis and the fetal skull are fixed. They’re not. Thanks to the openings in the fetal skull (fontanelles) and the unsealed joints between its bony plates, the fetal skull can mold to the mother’s pelvis. Furthermore, pregnancy softens maternal ligaments, enabling the pelvis to flex open. They also assume that birthweight relates strongly to skull circumference, but mostly, increasing birthweight just means plumper, not bigger headed. And they assume that maternal-fetal dimensions are the sole factors determining ability to pass through the pelvis, but unfavorable fetal positioning—head facing the mother’s belly (occiput posterior) or to the side (occiput transverse) or not chin on the chest (deflexed) or tipped to one side (asynclitic)—all play as big or bigger role by presenting a wider diameter to the pelvic inlet.

For another, the facts contradict their theory. Women having vaginal births after cesarean (VBACs) not infrequently deliver babies as big or bigger than the baby they were supposedly too small to deliver the first time. What’s more, the rise in cesareans for progress delay can’t be due to a surge in big babies because according to “Myth & Reality Concerning Cesarean Section in the U.S.,” there hasn’t been one. The proportion of babies weighing 8 lb 13 oz (4000 g) or more, the usual definition for macrosomia, has fallen from 10% to 8% since 1991 while the cesarean rate for babies of every birthweight soared.

So, let’s turn the spotlight on the real reasons behind the high cesarean rate for progress delay: obstetric practices and beliefs.

FACT: When obstetricians believe a woman won’t be able to birth her baby, it affects their decision-making in ways that tend to make it a self-fulfilling prophecy. For example, studies consistently show that if the doctor suspects a big baby, the woman is far more likely to have a cesarean than when the baby actually weighs in the macrosomic range, but the doctor didn’t suspect it.1,2,3,4,5,6,7

FACT: The cesarean rate for macrosomic babies has skyrocketed over time. It was 3% in 1958 in Great Britain.8By the 1990s, obstetricians might perform cesareans on as many as half of women with babies of this size.2,6Unless you’re prepared to argue that women’s pelvises have been shrinking over the decades, this means cesarean rates for big babies must be due to changes in their doctors’ thinking, not them.

FACT: Conventional obstetric management handicaps women, depriving them of an edge that might make a difference, especially if the baby is bigger. To name a few common practices that tilt the playing field toward cesarean:9,10

  • Inducing labor when the cervix isn’t ready;11
  • Admitting women to the hospital in early labor;12,13
  • Promoting universal use of epidurals;14
  • Keeping women in bed;15
  • Imposing overly restrictive time limits for making progress in early labor,16,17 during pushing,16,17 and when oxytocin is being given to strengthen contractions;18,19
  • Having women push on their backs.20

The enormous variation in cesarean rates in similar women makes clear that whatever is going on, again, it isn’t about women.21

Certainly, some women would be unable to birth their babies vaginally no matter what their care or how much time they were given. For these women, cesareans may be a lifesaver, but as one obstetrician summed it up:22

I can’t believe that evolution is pushing us into the operating room. I think we’re pushing ourselves into the operating room . . . it’s almost like the perfect storm. You’re going to pay me more, I get to worry less, you’re not going to sue me, and I’ll be done in an hour.

So, please, let’s stop blaming the victims for what is essentially the fault of their care providers.

Reblogged with permission from Henci Goer, a member of ICAN’s Advisory Committee, from her website Childbirth U.com. Childbirth U offers narrated slide lectures at modest cost to help pregnant women make informed decisions and obtain optimal care for themselves and their babies.

  1. Blackwell SC, Refuerzo J, Chadha R, et al. Overestimation of fetal weight by ultrasound: does it influence the likelihood of cesarean delivery for labor arrest? Am J Obstet Gynecol 2009;200(3):340 e1-3.
  2. Levine AB, Lockwood CJ, Brown B, et al. Sonographic diagnosis of the large for gestational age fetus at term: does it make a difference? Obstet Gynecol 1992;79(1):55-8.
  3. Melamed N, Yogev Y, Meizner I, et al. Sonographic prediction of fetal macrosomia: the consequences of false diagnosis. J Ultrasound Med 2010;29(2):225-30.
  4. Parry S, Severs CP, Sehdev HM, et al. Ultrasonographic prediction of fetal macrosomia. Association with cesarean delivery. J Reprod Med 2000;45(1):17-22.
  5. Sadeh-Mestechkin D, Walfisch A, Shachar R, et al. Suspected macrosomia? Better not tell. Arch Gynecol Obstet 2008;278(3):225-30.
  6. Weeks JW, Major CA, de Veciana M, et al. Gestational diabetes: does the presence of risk factors influence perinatal outcome? Am J Obstet Gynecol 1994;171(4):1003-7.
  7. Weiner Z, Ben-Shlomo I, Beck-Fruchter R, et al. Clinical and ultrasonographic weight estimation in large for gestational age fetus. Eur J Obstet Gynecol Reprod Biol 2002;105(1):20-4.
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  9. Boyle A, Reddy UM, Landy HJ, et al. Primary cesarean delivery in the United States. Obstet Gynecol 2013;122(1):33-40.
  10. Declercq E, Sakala C, Corry MP, et al. Listening to Mothers III. Pregnancy and Birth. New York: Childbirth Connection; 2013.
  11. Teixeira C, Lunet N, Rodrigues T, et al. The Bishop Score as a determinant of labour induction success: a systematic review and meta-analysis. Arch Gynecol Obstet 2012;286(3):739-53.
  12. Kauffman E, Souter VL, Katon JG, et al. Cervical Dilation on Admission in Term Spontaneous Labor and Maternal and Newborn Outcomes. Obstet Gynecol 2016;127(3):481-8.
  13. Tilden EL, Lee VR, Allen AJ, et al. Cost-Effectiveness Analysis of Latent versus Active Labor Hospital Admission for Medically Low-Risk, Term Women. Birth 2015;42(3):219-26.
  14. Bannister-Tyrrell M, Ford JB, Morris JM, et al. Epidural analgesia in labour and risk of caesarean delivery. Paediatr Perinat Epidemiol 2014;28(5):400-11.
  15. Lawrence A, Lewis L, Hofmeyr GJ, et al. Maternal positions and mobility during first stage labour. Cochrane Database Syst Rev 2013;10:CD003934.
  16. American College of Obstetricians & Gynecologists, Society for Maternal-Fetal M, Caughey AB, et al. Safe prevention of the primary cesarean delivery. Am J Obstet Gynecol 2014;210(3):179-93.
  17. California Maternal Quality Care Collaborative. Smith H, Peterson N, Lagrew D, et al. Toolkit to Support Vaginal Birth and Reduce Primary Cesareans: A Quality Improvement Toolkit. Stanford, CA: California Maternal Quality Care Collaborative; 2016.
  18. Rouse DJ, Owen J, Hauth JC. Active-phase labor arrest: oxytocin augmentation for at least 4 hours. Obstet Gynecol 1999;93(3):323-8.
  19. Rouse DJ, Owen J, Savage KG, et al. Active phase labor arrest: revisiting the 2-hour minimum. Obstet Gynecol 2001;98(4):550-4.
  20. Reitter A, Daviss BA, Bisits A, et al. Does pregnancy and/or shifting positions create more room in a woman’s pelvis? Am J Obstet Gynecol 2014;211(6):662 e1-9.
  21. Kozhimannil KB, Arcaya MC, Subramanian SV. Maternal clinical diagnoses and hospital variation in the risk of cesarean delivery: analyses of a National US Hospital Discharge Database. PLoS Med 2014;11(10):e1001745.
  22. Kennedy HP, Doig E, Tillman S, et al. Perspectives on Promoting Hospital Primary Vaginal Birth: A Qualitative Study. Birth 2016;43(4):336-45.

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