Another post about twins and IVF. I’ll back off soon, I promise. A systematic review published just last year notes that the outcomes of twins conceived after IVF is worse than those who are conceived naturally. (Rossi AC, D’Addario V. Neonatal outcomes of assisted and naturally conceived twins: systematic review and meta-analysis. J Perinat Med. 2011;39(5):489-93. http://www.degruyter.com/view/j/jpme.2011.39.issue-5/jpm.2011.058/jpm.2011.058.xml)
One of the problems with doing this kind of analysis is that twins after IVF are almost all fraternal, rather than identical. Identical twins that are monochorionic almost all have anastomoses in the placental circulation, which is probably the main cause for the increase in morbidity and mortality among identical twins, compared to fraternal twins.
So to look at the outcomes of twins after IVF you need to compare just non-identical twins, which is not necessarily obvious at birth, unless the twins are of different sex. (There are a few identical twins after IVF, the frequency of identical twins is increased compared to spontaneous twins, I guess messing around with embryos makes them a bit more likely to divide in two).
When you do this analysis you find that there are more twins born prematurely after IVF, there are more born extremely prematurely, there are more very low birth weight, and there is more perinatal mortality among IVF twins than among non-identical twins conceived spontaneously. And of course all of those outcomes are much higher than among singeltons, either IVF or spontaneously conceived.
Why is this? Well there is some data that primigravidae who have twins deliver earlier than multigravidae. (That is even true for women having their first baby when they are just having one, the average duration of the first pregnancy is about 1 week more than the average duration of subsequent pregnancies). So the major difference between women having twins after IVF and those with a spontaneous conception, is principally because most of the IVF mothers have not had a baby before, so they deliver even earlier, and have more complications of prematurity.