Why are some chromosomal abnormalities (Downs Syndrome) allowed to continue through a normal pregnancy where almost all other chromosomal abnormalities are miscarried?

Question

How does the body know which chromosomal abnormality is good enough to become a person and others are better to be ended?

In: Biology

Answer 1

It isn’t like the body checks whether the baby will be able to live long before it decides to give birth. It is just that some anomalies are so detrimental to life that the fetus/embryo just starts to die.

The mother’s body recognizes the dying offspring and flushes it out to prevent damage to the uterus.

But Down Syndrome children are physically viable. Their heart works, their blood vessels work, etc. To the mother’s body, it is perfectly normal. It cares not how smart or capable the child will end up being.

And even with that basic check, sometimes it fails, with some mothers giving birth to dead children (1 in 160 “completed” pregnancies are stillborn in the US)