Study Finds Why is it so Difficult for Humans to have Baby

According to a new study by a researcher at the Milner Centre for Evolution at the University of Bath contends that 'selfish chromosomes' are to blame for the early demise of the majority of human embryos. The discovery explains why human embryos frequently don't survive while fish embryos do not have repercussions for the management of infertility.
The findings of the research were published in the journal 'PLoS Biology'. About half of fertilised eggs die very early on before a mother even knows she is pregnant. Tragically, many of those that survive to become a recognised pregnancy will be spontaneously aborted after a few weeks. Such miscarriages are both remarkably common and highly distressing.
Professor Laurence Hurst, Director of the Milner Centre for Evolution, investigated why, despite hundreds of thousands of years of evolution, it's still so comparatively hard for humans to have a baby.
The immediate cause of many of these early deaths is that the embryos have the wrong number of chromosomes. Fertilised eggs should have 46 chromosomes, 23 from mum in the eggs, and 23 from dad in the sperm.

Professor Hurst said, "Very many embryos have the wrong number of chromosomes, often 45 or 47, and nearly all of these die in the womb. Even in cases like Down syndrome with three copies of chromosome 21, about 80 per cent sadly will not make it to term."
Why then should gain or loss of one chromosome be so very common when it is also so lethal?
There is a number of clues that Hurst put together. Firstly, when the embryo has the wrong number of chromosomes it is usually due to mistakes that occur when the eggs are made in the mother, not when the sperm is made in the father. In fact, over 70 per cent of eggs made have the wrong number...