Why it is so hard for humans to have a baby?

From Phys.Org:

New research by a scientist at the Milner Center for Evolution at the University of Bath suggests that “selfish chromosomes” explain why most human embryos die very early on. The study, published in PLoS Biology, explaining why fish embryos are fine but sadly humans’ embryos often don’t survive, has implications for the treatment of infertility.

About half of fertilized eggs die very early on, before a mother even knows she is pregnant. Tragically, many of those that survive to become a recognized 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 Center 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 much of these early deaths is that the embryos have the wrong number of chromosomes. Fertilized eggs should have 46 chromosomes, 23 from mum in the eggs, 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% 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?

More here.