I don't think the UN team was able to attribute any child birth defects to increased radiation. The horror books with pictures of them are shock collections of deformed children. It's possible that the rampant alcoholism of the latter Soviet era was the likely cause.

Alcoholism causes birth defects? How?

A simple google search will bring up hundreds of articles linking alcohol consumption during pregnancy to birth defects. It is a well established relationship.

In the US we even have warnings on all alcoholic beverages that say pregnant women should not drink alcohol.

I looked at the CDC's page (https://www.cdc.gov/ncbddd/fasd/alcohol-use.html). It contains a number of very negative effects from drinking alcohol while pregnant. But because the context was birth defects caused by radiation, I was thinking more in terms of visible deformities - which can be caused by radiation, but which are much less significant from alcohol.

Perhaps I was being too limited in my concept of "birth defects" - including for radiation. If the radiation while pregnant (or before) gives the child cancer at five, is that a "birth defect"? Maybe not, in the way I was thinking, but I can see how it might be thought of that way.

What childbirth defects?