One worry common among pregnant mamas is that they will grow a baby “too big to birth (naturally).” We’ve all heard the horror stories – and maybe seen the photos on the internet – of 10, 11, or 15-pound whoppers of newborns. And most of us receive at least one third-trimester ultrasound, a standard part of which is the estimation of fetal size. So, just what are the risks of a large baby, and how should we proceed?
First, we need to know what constitues a “big” baby. There are two kinds of “big”: macrosomia and large for gestational age. Macrosomia, or “large body,” can be alternately defined as a birth weight greater than 8lb 13 oz (4000g) or one greater than 9lb 15oz (4500g). Slightly over 7% of American babies are born greater than 4000g, and slightly over 1% weigh more than 4500g at birth. Large for gestational age (LGA), on the other hand, is the medical term for any baby born over the 90th percentile for babies born on the same day of pregnancy (how many weeks and days pregnant mama is when baby is born). By definition, 10% of all babies are LGA.
Your baby’s birth weight is influenced by many factors, including maternal and paternal height, whether mom was obese prior to pregancy, mom’s weight gain during pregnancy, baby’s sex, and chemical exposures including smoking. By far the biggest risk factor for unusually (and unhealthily) large babies, though, is gestational diabetes. Like other pregnancy complications, gestational diabetes carries its own list of risks for both mom and baby, and is to be taken seriously.
However, even in the uncomplicated pregnancy there are certain complications associated purely with fetal size. Most of these are a result of the increased mechanical demands of a larger baby passing through the birth canal. Labor is more likely to stall, vacuum or forceps delivery is more often necessary, and perineal tearing and pelvic/tailbone trauma are more common. The increased risk most cited by physicans is also a mechanical complication: shoulder dystocia, which is quite simply the baby’s shoulders getting stuck behind the mother’s tailbone after the head is born. Although shoulder dystocia is more common among large babies, it is not altogether uncommon in babies of any size, and it can nearly always be managed with relative ease when it does occur. (And, lest this all seem rather bleak, there is some good news for parents of larger than average babies: big newborns – up to about 10lbs – grow up to be more successful grade school students.)
Thus it would seem that prenatal diagnosis of fetal size would be useful information for a birth team, right? After all, surely that is the reason two-thirds of US women receive late-pregnancy u/s to estimate size. Well, yes and no, for it turns out these estimates are notoriously inaccurate. Of the many formulas used to estimate birth weight, the very best formula is within +/-10% of baby’s weight only 70% of the time. All formulas work best on mid-sized babies. Using a little back-of-the-envelope math, this means that, if you’re told your baby is “large” or “very large”, there is at least a one-in-three chance that your baby’s real weight is a whole pound different than the estimate you’ve been given. In fact, with most methods there is little better than a 50/50 chance that your “suspected large” baby is in fact macrosomic – about the same accuracy as a physican’s guess after feeling the mother’s belly, and only moderately more accurate than just guessing every baby at 3455g (7lbs 10oz, or perfectly average).
As unreliable as fetal size estimates are, however, they can have a marked impact on pregnancy and labor management. As many as two out of three physicans will propose induction, and as many as one in three will propose scheduled cesarian, on the basis of such an ultrasound finding. Additionally, doctors are more likely to diagnose the labors of suspected large babies as failing to progress, thereby setting off a string of interventions to speed/assist the birth, even when those labors are in fact progressing at the same speed as labors without suspected large babies. Comparing labors in which a baby that was both suspected to be and actually large with labors in which an actually large baby was not previously suspected, the suspicion of macrosomia markedly increases medical intervention without significantly changing outcomes. The American College of Obstetricians and Gynecologists specifically names macrosomia as an insufficient reason to induce birth (or schedule a cesarian) prior to 39 weeks gestation.
So, while it is certainly true that big babies can be hard to birth, it is not necessarily the case that estimated fetal size alone should alter an expecting mother’s plans. After all, babies of any size can be difficult to birth. Quite often the culprit in a difficult birth isn’t baby’s size so much as baby’s positoning. The strategies for vaginally birthing a large baby are really the same as those for any other baby: using good posture through late pregnancy, and utilizing motion and gravity during labor. Certainly, it is true that a large baby has less room to maneuver during birth, which makes such positioning tactics even more important.
Whether your baby is large or small, your doula can help you try out labor positions and techniques including walking, dancing, kneeling, leaning, lunges, and squatting. Even with an epidural, periodic position changes help the baby ease its way down the birth canal and into the world.
Dekker, Rebecca. “What Is the Evidence for Induction or C-Section for a Big Baby?” Evidence Based Birth. http://evidencebasedbirth.com/evidence-for-induction-or-c-section-for-big-baby/
Nahum, Gerard G, MD, FACOG. “Estimation of Fetal Weight.” Medscape. http://emedicine.medscape.com/article/262865-overview#aw2aab6b2
“The Dangers of Estimating Fetal Weight Near Term.” The Well-Rounded Mama. http://wellroundedmama.blogspot.com/2012/09/the-dangers-of-estimating-fetal-weight.html
“Optimal Fetal Positioning.” Spinning Babies. http://spinningbabies.com/about-spinning-babies/optimal-fetal-positioning
Leonhardt, David and Amanda Cox. “Heavier Babies Do Better in School.” The New York Times. http://mobile.nytimes.com/2014/10/12/upshot/heavier-babies-do-better-in-school.html?referrer&_r=2