Twinning

Because it's come up in a couple of Facebook comments, I want to say a word or two about the upper bound on the number of babies Sarah might be carrying. 

The short answer is: two.  The doc transferred two embryos.  If they've both survived, then Sarah's carrying twins; if only one has survived, then it's just one baby (for this time).

The longer answer is: well, it depends.  There were two blastocysts on Day 5.  Most identical twinning is thought to happen at the blastocyst stage between Days 4 and 8. The astute reader will note that 5 is more than 4 but less than 8; this means it is at least possible for a day Day 5 blast to split into monozygotic (identical) twins after the transfer.  So it's possible, in the strictest sense of the word, that Sarah's carrying quads, in the form of two sets of identical twins.

Setting aside what is and isn't possible, the likelihood of Sarah carrying one, two, or more babies is subject to the usual rules of probability.  On transfer day, Dr. Kutteh told us that the probabilities broke down by thirds: one-third of women in Sarah's situation would have twins; one third would get a single pregnancy; and one third would get no pregnancy.  Two weeks on, we know we're not in the "no pregnancy" group.  So there's a ~50% chance that we're in the singleton group, and a ~50% chance we're in the dizygotic twins group.

Beyond that, probabilities get small. Identical twins account for about 3 of every 1000 pregnancies.  That's 0.3%.  So the probabilities break down like this:

49.85% one baby
49.85% fraternal twins
0.15% identical twins
0.15% triplets, composed of 1 set of identical twins and a fraternal triplet.

The probability of quads of any kind is too small to register at this number of sig figs.  And of course the identical twinning numbers here are too big: some identicals form between days 1 and 5, and we know that didn't happen.  But this is a good rough guide to the possibility of tour taking home more than two babies this go-round. 

The take home message is: because identical twinning is always rare the odds are slim; but because identical twinning does happen and can't be ruled out on the basis of the transfer day, the odds are not zero.