In the 1930s a famous antibacterial drug was invented in Germany called Sulfanilamide. The drug was used for treating infections. Sulfanilamide was tested on pregnant rats. Testers were able to show that the drug had a damaging effect on the fetuses of those rats (1939).
The idea that a drug could pass the placenta and damage a fetus was therefore well known and well publicised, including in Germany.
Source: Speert, Harold: The placental transmission of Sulfanilamide and its effects upon the fetus and newborn, John Hopkins University, 1940 (submitted in December 1939).