Came across an article today from the UK which discusses some research that suggests that IVF treatment is more successful in the summer months.
They found that women (in their study) tend to need fewer drugs to ovulate and have a higher rate of achieving pregnancy. The reasoning is that mammals are ruled by a "biological process" to give birth in spring and allow the newborn to mature for a couple of months before having to survive through the winter - in the same way that other mammals do.
It's an interesting concept, although the researchers do say that you shouldn't put off your IVF treatment until summer. After all that may be 6 months in the future and with time being of the essence when it comes to fertility it would be unwise to wait longer than necessary.
I seem to have fallen into the success in summer catergory by accident. My FET (after an unsuccessful IUI in winter and an unsuccesful IVF in spring) was in the height of summer and my son was then born in the middle of spring. But, of course, I do know a whole lot of other IVF babies born in winter.