Does radiation poisoning cause birth defects?

Yes. It depends on the amount of exposure and the timing in the pregnancy. Early on in the pregnancy, radiation poisoning will likely lead to a miscarriage. Later on in the pregnancy, radiation poisoning can lead to death or growth disturbances or congenital malformations.
Yes. This depends on how early in pregnancy the exposurem occurs, and how much radiation the pregnancy is exposed to. The effect can be as severe as a complete miscarriage, if the exposure is early in pregnancy, or little to no effect if exposure occurs later.