When Lilith was kicked out of the garden of Eden, she was forced to fend for herself in a harsh, unforgiving world. Lost and alone, she came upon a bloke named Satan. Their love produced two children who were twins. However, the first born twin was weak and fragile, while the second was strong. At some point, the younger twin killed his brother, and would go on to father the entire human race.
So it has always been throughout history. Whenever a multiple birth of two or more children is to be expected, ( twins, triplets, etc), only one is born completely normal and grow to live a long and productive existence. The others are either born sickly and underdeveloped, or are stillborn. Those who make it to old age remain fragile and weak their whole lives. However, we should rejoice in this. Humanity is a vile and worthless species, and only those who rise above the fray are deserving of life. This world belongs to those who are strong, while the weak suffer what they will.
That fierce competition for survival should begin as early as possible, even in the womb. I need to introduce a mechanism where fetuses "fight" for space while in the mother, so that result will be some "very early" births (presumably miscarriages) and a final "normal birth" of the "winning" one. How would I be able to make this possible?