Radiation activated chemicals
During pregnancy, some chemicals are needed for the fetus to develop correctly. Unfortunately, these chemicals suffer rapid degradation when exposed to radiation and if the mother does not produce them in high amounts, the fetus dies. Once the fetus stops needing them, it is in the safe zone and develops without problem. You can choose how long this period is to help in the narrative.
To solve the problem of why this species has not gone extinct, this chemical could be the one that grants the specimens radiation resistance and DNA repair capabilities.
Also if so many miscarriages happen you would need one of these circumstances to keep the species alive:
- High rate of pregnancies.
- Long lifespan.
- Lots of specimens solely dedicated to breeding.
And with time, evolution would select those who produce more of this chemical and the effect would mitigate. There are also protections against this, like moving all pregnant women to living quarters made of lead or things like that.
High activity nebula
Imagine a planet near a high activity stellar zone, where a lot of supernovas happen regularly (in cosmic time scale). This would mean that the planets formed in these nebulas would have a high percentage of heavy elements, some of which are radioactive.
Also, the comets flying around these zones would have a high quantity of radioactive metals. If one or some of these comets fell into the planet and burned out in the atmosphere, all of these metals would be spread out through the atmosphere and around the world, which would make the surface of the planet quite radioactive, plus activating some of the elements already present in the planet.