If you're squeamish, don't read on, or look at any of the websites I reference. You WILL regret it.
It should be noted that all radioactive chemicals are teratogenic. But...how?

Well, foetuses have rapidly dividing cells. Cancer is a genetic malformation in rapidly dividing cells, by definition. So, foetus cells can be affected by radiation. Or not...once the foetus is past a certain age, you'll most likely be OK, as the genetic code for these dividing cells is more or less set.

But, if you get your genetic code messed up at a very early stage, say at the zygotal stage, where there's only a few cells, you will have problems.

Occasionally, the body will detect the abnormality, quietly think "Oh bugger" to itself and miscarry the zygote. If not, there's all sorts of nasty repercussions. Like the baby being stillborn. And not only will it be stillborn, but because the genetics was mucked about with at such an early age, it will be grossly deformed. The mother would probably have severe psychological problems as a result of this.

And though it may be nasty to think about and quite insensitive to say so, biologically a foetus affected by a nuclear accident is essentially a tumour. It's cancer taken up twenty-five notches. And it's common.

Chernobyl has had plenty of problems of this nature, as has Mayak and other places affected by nuclear pollution. In Mayak there have been terrible problems because of local people bathing in contaminated water. More information in the Mayak node or at http://archive.greenpeace.org/mayak.