The Mother's Womb Tests the Viability of the Fetus

Given the invasive nature of pregnancy, it’s perhaps not surprising that the primate womb has evolved to be wary of committing to it. Mammals whose placentae don’t breach the walls of the womb can simply abort or reabsorb unwanted foetuses at any stage of pregnancy. For primates, any such manoeuvre runs the risk of haemorrhage, as the placenta rips away from the mother’s enlarged and paralysed arterial system. And that, in a sentence, is why miscarriages are so dangerous.

It’s also why primates make every effort to test their embryos before they allow them to implant. The embryo is walled out by the tight-packed cells of the endometrium, while an intimate hormonal dialogue takes place. This conversation is, in Haig’s words, a ‘job interview’. Should the embryo fail to convince its mother that it is a perfectly normal, healthy individual, it will be summarily expelled.

How does an embryo convince its mother that it is healthy? By honestly displaying its vigour and lust for life, which is to say, by striving with all its strength to implant. And how does the mother test the embryo? By making the embryo’s task incredibly difficult. Just as the placenta has evolved to be aggressive and invasive, the endometrium has evolved to be tough and hostile. For humans, the result is that half of all human pregnancies fail, most at the implantation stage, so early that the mother may not even realise she was pregnant.

Embryonic development becomes a trial of strength. And this leads to another peculiarity of the primate reproductive system – menstruation. We have it for the simple reason that it’s not such an easy matter to dispose of an embryo that is battling to survive. The tissues of the endometrium are partially insulated from the mother's bloodstream, protecting her circulatory system from invasion by a placenta she has not yet decided to accept. But that means her own hormonal signals can struggle to be heard inside the womb. So, rather than risk corruption of the endometrial tissue and ongoing conflict with an embryo, what does the mother do? She just sloughs off the whole endometrium after each ovulation. This way, even the most aggressive embryo has to have her agreement before it can get comfortable. In the absence of continual, active hormonal signalling from a healthy embryo, the entire system auto-destructs. Around 30 per cent of pregnancies end this way.