Q. Is there really a way to accurately estimate the weight of a baby while in utero? If so, how is it figured? I've heard most estimates by ultrasound in the third trimester are inaccurate.
A. The two common ways are with ultrasound and with a good midwife's hands. The ultrasound uses a computer program that combines measurements such as head size, abdominal circumference and leg length to calculate an estimated weight. As a midwife, I think of a five-pound bag of sugar and a ten-pound bag of sugar and go from there. Every once in a while, I totally blow it, but I'm usually within 4 ounces--much better than the ultrasound! I was recently pressured to immediately induce a woman for a "10 lb 12oz" child (according to the ultrasound) which turned out to be 9lbs 1oz, similar to her other children. My most recent "goof" was a woman whose largest prior baby (#3) was 8 lbs 4oz. I thought #4 was larger, maybe 9 or so pounds. She was 10 lbs 7oz! Which is a long way of saying, if you want to know how big the baby is, wait until it's born and weigh it. There's no sure way to tell ahead of time.
On the other hand, if what you're really asking is how we can tell whether your baby is "too big" to deliver vaginally, the only real way to tell is to try it. The true story I tell healthy women who are worried that their babies are getting "too big" is of a teenager having her first baby who weighed 100 lbs soaking wet (size zero jeans) when I first met her. She weighed 140 lbs and measured 42cm the day before she delivered, and was "all baby." She did have a long labor, about 24 hours, without medication. When the head delivered, it was literally twice as big as her upper thigh. We were all set for problems with the shoulders, but she just pushed her baby out without even a tear, at which time we all nearly fainted. He was 10 lbs 1oz--more than 10% of her weight. Then she admitted she had been eating cookies and cake for about two months.
If we had done a cesarean for "failure to progress" as fast as usual, no one would have faulted us; how can a 100 lb woman deliver a first baby more than 10 lbs? But think of what we would have taken away from that mother--those bragging rights are priceless! What a woman/mother she became that day, she was so proud of herself and rightly so. At four months, with only breast milk, the child weighed 25 lbs--one-fourth of her weight. They are hysterical to see together--he looks bigger than she does. But after that, I would give ANY woman who wants to, a try at labor.