Question: Why do some embryos develop abnormally?

Paul McKeegan answered on 7 Nov 2017:

A good question and one with lots of answers!
They could have an incorrect number of chromosomes, which carry DNA. These embryos can develop for a few days but fail to form blastocysts or to implant to the womb.
They could be fertilised by multiple sperm, known as polyspermy. Again, they can develop for a few days as embryos initially use the proteins provided by mum, but once they need to start making proteins using their own genome, they stop developing.
Then there are embryos which develop further, but something slightly different happens during foetal development. There are very many possibilities here, really minor ones like a cleft palate, which is very common and often treated with surgery, or polydactyly, which means bonus fingers, or syndactyly, which is fused fingers. There are very serious and life-threatening ones too. Some abnormalities are caused by genetics, and others are influenced by diet or health. For example, diabetic mothers are more likely to have very large babies. Some are caused by poisons, like the thalidomide children in the 50s – ask your science teacher about this one, it’s usually in the GCSE/A level syllabus. Essentially a new wonder drug turned out to have unwanted effects in pregnant mothers, leading to children with a variety of conditions. Thankfully drug production and trials are much more strongly regulated now!