"Given the myriad steps involved in fetal growth, each presenting the opportunity for developmental mischief, it is not surprising that more than 7,000 kinds of birth defects are known to occur. The causes of birth defects remain largely a mystery, however, although a few culprits have been identified as important contributors, including some environmental agents. ... In many regions birth defects are the largest single cause of infant deaths.
But reliable numbers for both defects and cases remain unclear, due in large part to a lack of funding, coordinated monitoring efforts, and adequate data. The causes of only about 30% of birth defects are somewhat well understood, and knowledge even of those is sometimes spotty. The 70% still unknown leaves open the possibility that environmental factors could play a significant role."