Question: Hi Paul, I would like to know how can a Single ball of cells can divede identically to form twins? And why do baby’s come out sometimes attached to each other?

Paul McKeegan answered on 7 Nov 2017:

Cool question! So identical twins happen about 0.3% of the time, or 3 in 1000 births. As you say, a single embryo (one egg fertilised by one sperm) splits into two to form two identical embryos. The two embryos share a placenta as they develop. It’s hard to study why exactly this happens, as it happens before pregnancy is easily detectable, in fact even before the embryo attaches to the womb.

The embryo initially is a ball of cells, dividing once every 18 hours or so but not changing overall size. When the embryo gets to about 100 cells, it forms a blastocyst. The cells start to pump water into the centre of the embryo, pushing the cells out to line the outer wall of the embyro. Most of the cells form a one-cell-thick layer around the fluid. These are called trophectoderm and go on to become the placenta and umbilical cord. Some of the cells form a clump together, called the inner cell mass. These go on to form the foetus, and eventually the baby! So very occasionally, 2 inner cell masses must form, and will develop as separate individuals as the embryo grows into 2 foetuses.

In the lab, we see that blastocysts very often collapse, with the fluid leaking back out, but then fill up again. In IVF clinics we often use time-lapse systems, which take pictures of the embryos every few minutes so you can see exactly what they are doing. These images show that blastocyst collapse and recovery is very normal. So the main idea at the moment is that sometimes, when a blastocyst collapses, the re-filling of fluid and expansion of the blastocyst causes the inner cell mass to split into two. This would then lead to identical twins!

Sorry, that was a long answer! Hope it is OK?