Reproduction

Once a sperm has fertilized an egg, the egg's wall cannot be penetrated by other sperm, limiting about 4 million sperm that may have started the journey towards the egg to 1 sperm. Occasionally the fertilised egg will split in two or another sperm will fertilise another egg resulting in twins and on very rare instances triplets or a higher number of eggs are fertilised. However an ordinary pregnancy, if any could be described as such, would be where the egg is fertilised by one sperm thus allowing human population to scale in proportion to physical human capacity.
We are one the most complex of species on the planet with physical and psychological needs and so Allah has not overburdened Man or placed an impossible task upon us by allowing all sperm to fertilise all eggs. In fact He has deliberately put factors in place to govern the personal well-being of parents and children and inhibited exponential population surges. However these factors do not inhibit serial reproduction after puberty. For example the head of a sperm contains an acrosin enzyme that dissolves the ovum casing of a woman's egg enabling the sperm to penetrate it. Once a sperm has fertilized an egg, the egg's wall cannot be penetrated by other sperm. The unsuccessful sperm die after about three days even though they have managed to get to the egg. Fertilisation has been pre-determined up to 4Million:1. Another example is of the female ovary, It may have millions of eggs but it will only ovulate one egg every month and stop ovulating whilst pregnant thereby restricting the number of eggs that can be fertilised at any one time. Breast-feeding also often inhibits pregnancy and so these factors govern pregnancy intervals and well-being. The end result is that the specie propagates, survives and inherits the world for the purpose it was created.