Anecdote time:
Way back in 1980, ultrasounds were not routinely used. Mom was not considered high-risk -- she was only 28, she had an easy delivery with my older sister, and didn't smoke, drink, or do drugs while pregnant with me. I was two weeks late, and Mom had been having very severe Braxton-Hicks contractions for three weeks prior. But that's pretty normal. She hadn't had any bleeding or spotting during those three weeks.
This was in the day of fathers sitting out in the waiting room, not being in the delivery room. My father and my grandfather were sitting there, watching TV and trying not pace the floors waiting. After several hours, one of the doors to the waiting room flew open, and the OB who was on call that night for the clinic Mom went to ran up to Dad and handed him a paper and pen. That doctor was an immigrant from Korea, and normally spoke very clearly with little accent, but he was talking *really* fast and the only words Dad could make out were "transfusion", "surgery", and "sign".
Dad signed, and the doctor yanked the paper out of his hand and ran back out of the waiting room through the other door. My grandfather reached into his shirt pocket, got his bottle of Valium (he had spasmodic toricollis, it was prescribed as a muscle relaxer back then), and handed my father a 10mg tablet. He said, "Here, I think you're going to need this" -- and then took two himself.
The way Mom tells it, she was alone in the delivery room. My grandmother was at home taking care of my older sister. She felt a sharp pain that was different from the contractions and some wetness, and thought it was her water breaking. She hit the call button, the nurse came to take a look and screamed. It wasn't just her water breaking -- she was hemorrhaging, and the nurse could see the umbilical cord in the vaginal vault.... not prolapsed, but ruptured.
We both made it, and were damn lucky. We both required blood transfusions, and they knew I had been without oxygen for at least four minutes. They told my parents there was a high likelihood that I would have brain damage. Why? The placenta had attached in the lower part of the uterus, fairly close to the cervix. The cord was not attached properly to the placenta, it was pretty much only connected with blood vessels and more to the membranes, and those vessels crossed over part of her cervix. When her water broke it ruptured the vessels, detached the cord from her placenta, and the placenta started to abrupt. That's why we were both bleeding. She had vasa previa as well as marginal placenta previa.
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Cord accidents kill a lot of kids, and placenta problems kill a lot of women as well as their babies.
When vasa previa is diagnosed by ultrasound before birth, there is only a 4% probability that the baby will die. The fetal mortality rate for undiagnosed vasa previa has ranged from 56% to over 90% in various studies. But it is easily detected by color Doppler ultrasound or the "3-D" ultrasound. Ultrasound can detect placenta previa as well, but early in pregnancy the false-positive rate is very high. In order to detect most cord and placental conditions, an ultrasound must be done in the third trimester. Vasa previa occurs in 1 out of every 3000 births, placenta previa is a 1 in 200 risk, but neither are associated with other fetal abnormalities and there are few reliable blood markers or risk factors to use to predict them.
Ultrasound is the best way to detect both conditions, and both are easily managed by scheduled Cesarean delivery. Ultrasound can also detect true knots, nuchal cords, and other cord problems. No medical procedure is ever 100% safe, but when more than one in 3000 babies are dying from ultrasounds, I'll join the crusade. Not until then, tho. They save lives. I was damn lucky to not be dead or a vegetable.