Dose limits for pregnant workers are not based on observed effect levels.
In the dose limits for radiation workers, the reason that the limits for pregnant workers are low is not based on the somatic effects on the fetus. Rather they were determined based on the idea that the fetus is regarded as a member of the public. The fetal exposure while the worker is pregnant is set so that the maximum dose is 1 mSv. An exposure dose with consideration to fetal effects would involve a fetal exposure of 100 mSv or higher. (See 1. Effects of Radiation on People.)