More than one in ten couples find it difficult or impossible to naturally conceive a baby. While infertility can be heartbreaking, in many cases it's not clear exactly what the problem is at a biological level. One of the most critical steps is when the embryo – a tiny ball of cells at this point – embeds itself in the wall of the womb. Around three quarters of pregnancies are thought to fail at this stage, and researchers are trying to find out why. One key player in this process is a protein called Alk2, shown in green in these images of a female mouse's womb shortly after an embryo has started to bed down. If Alk2 is missing, then it can't embed properly and there will be no mouse pups. Unpicking the molecular threads involved in early pregnancy could bring hope to infertile couples in the future.
Written by Kat Arney
BPoD stands for Biomedical Picture of the Day. Managed by the MRC London Institute of Medical Sciences the website aims to engage everyone, young and old, in the wonders of biomedicine. Images are kindly provided for inclusion on this website through the generosity of scientists across the globe.