Why would practitioners want to induce labor for suspected big baby?
The theory goes that inducing labor will prevent:
Cesareans, due to the baby growing too large to fit through the pelvis.
Shoulder dystocia, a situation where the head is born, but the shoulders hang up behind the pubic bone.
Birth injuries, namely, broken collar bone, or injury to a complex of nerves controlling the shoulder and arm (brachial plexus injury). Birth injuries often, though not necessarily, occur in conjunction with shoulder dystocia.
However, studies consistently show that inducing labor for suspected big baby accomplishes none of the above (2-3,5,7,9-10,15-16). These studies include two trials, randomly assigning women thought to have big babies either to induction or to await spontaneous labor (7,15). Random assignment trials produce the strongest evidence because they eliminate bias by ensuring that the two groups are truly similar.
Why doesn’t inducing labor help?
Whether a woman carrying a big baby has a cesarean depends largely on her caregiver’s management, not her pelvis. In proof of this, the cesarean rate for babies weighing 4,000 grams or more was three percent in 1958 in Great Britain (4). These days, U.S. obstetricians may perform cesareans on as many as half of women with babies of this size (11,16).
The practitioner’s belief that women cannot safely birth big babies vaginally, or cannot birth them vaginally at all, will lead to cesarean sections. Several studies illustrate this factor at work. Studies have found that:
-- C-section rates for mothers having big babies can vary enormously among individual practitioners. One study reported that having a private obstetrician tripled the odds of cesarean compared with having a resident or a midwife (2). Another found rates among obstetricians ranging from less than five percent to one-third (14).
-- Doctors may be likely to order planned cesareans for women suspected of carrying big babies. This was true for one-third of the women in one study (3).
-- When obstetricians believe that women are carrying a big baby, far more of them may have cesareans than when doctors don’t think the baby is that big, but it actually is. A study reported that half the women whose babies had estimated birth weights of 4,000 grams or more had cesareans versus less than one-third of women with lower estimated birth weights but whose babies were just as big (16).
-- The reverse is also true. When ultrasound predicts a big baby, women may be just as likely to have a cesarean when the ultrasound is wrong than when it is right. In yet another study, roughly half the women predicted to have babies with birth weights in the top ten percent had cesareans regardless of whether their babies actually weighed in this range (11).
-- Doctors may not give women a fair chance to labor when they think the baby is big. A study found that when obstetricians thought the baby would weigh 4,200 grams (9 lbs. 4 oz.) or more, half the cesareans for poor progress were performed in early labor (16). When they didn’t think the baby was that big, although it was, they performed cesareans for poor progress in early labor less than twenty percent of the time.