When a bull calf and a heifer calf are born as twins, there is a higher than 90 per cent chance of the heifer being infertile – the heifers are known as freemartins.
This happens when the twins’ blood supplies fuse together during early pregnancy and are shared by the calves, allowing for an exchange of cells and hormones between the two foetuses.
Sexual development in the heifer calf is inhibited by the presence of male cells and hormones.
The ovaries fail to develop in a fully-grown freemartin cow.
She fails to cycle and never becomes pregnant.
The uterus is also underdeveloped and the udder and teats are small.
A freemartin appears like a normal heifer, therefore you need to get a veterinarian to diagnose her as a freemartin.
There is no treatment for freemartin cows; the best strategy is to permanently identify all heifer calves born twins with a bull and assume they will be infertile.