Critical Thinking About Preterm Survival Rates

So, I’ve been vaguely aware of lots of claims that survival of babies born before 24 weeks has dramatically improved recently. This matters because the current standard UK cut-off for abortions is 24 weeks. (Later abortions are permitted in special circumstances.) The forced-childbearing lobby (considerably weaker in the UK than in the US) is making heavy use of these claims of better survival before 24 weeks to argue for an earlier cut-off. The Guardian’s excellent Bad Science column does a beautiful job on these claims of increased early survival rates. First, these claims represent a distinctly minority viewpoint:

Scientific and medical expert bodies giving evidence [before the parliamentary committee] say that survival in births below 24 weeks has not significantly improved since the 1990s, when it was only 10-20%. But one expert, a professor of neonatal medicine, says survival at 22 and 23 weeks has improved. In fact, he says survival rates in this group can be phenomenally high: 42% of children born at 23 weeks at some top specialist centres. He is quoted widely: the Independent, Telegraph, on Newsnight, by Tory MPs, and so on. The figure has a life of its own. 

The key to understanding the 42% claim is that it matters which babies are being looked at. Is it 42% of babies born alive, or 42% of babies admitted to intensive care? As Ben Goldacre writes,

To be admitted to neonatal ICU, the doctors have to think you’ve got a chance. Often you have to be transferred from another hospital, in an ambulance, and for that you really do have to be more well. Therefore, if your denominator is “neonatal ICU admissions”, your survival rates will be higher, but you are not comparing like with like. 

A bit of searching around reveals that the apparent increase in 23 week survival was due to looking not at all babies born alive (the relevant figure if what you’re wondering is how well babies at this age can survive outside the womb), but rather all babies admitted to intensive care.This is a really important distinction, and very likely to be important in discussions outside the UK as well.