Perhaps the most interesting example of unintended adverse consequences in obstetrics is the history of the use of diethylstilbestrol (DES) (18, 19). 

Reference 18 is a Centers for Disease Control (CDC) summary with relevant links.  The drug was prescribed to prevent miscarriages; although it was shown to be ineffective in 1953 it continued to be used until the unusual complication of clear cell adenocarcinoma of the vagina in some of the daughters of women who had taken the drug in pregnancy was recognized in 1971.  If this distinctive adverse effect had not occurred when, if ever, would the increase in more commonplace problems of infertility and complications of pregnancy in women who had been exposed as fetuses have been recognized?  A reliable source tells me that she heard a radio interview some years ago in which it was mentioned that the first recognition of the DES-carcinoma of the vagina association was by a group of mothers of affected daughters conversing in an elevator and not by the preceding medical investigational interviews.

Reference 19 has an ad from 1957 recommending one brand of DES for all pregnancies.