Some interesting facts that may or may not bear on this discussion:
When human females are born, their ovaries already contain all the eggs they will produce in their lifetime. These number about 200,000 to 300,000 eggs and all form in the seventh and eighth month of gestation. During their approximately 30 fecund years they will release only about 400 of these eggs for potential fertilization, so over 99.9% of the eggs are "wasted". It is not just the vast majority of male sperm, which are produced continuously during adulthood, that go unused;
Approximately 65% of all the released eggs that get fertilized abort naturally due to various causes: fail to implant, the egg or sperm is dysfunctional, the uterine lining rejects the egg, and other causes. Thus, nature (or god) can be seen to be an avid abortionist;
Probably a little off topic, but still very interesting: When a woman gives birth to identical twin boys or girls, she is said to have two sons or daughters. This is not accurate. She has a son and a grandson or a daughter and a granddaughter. The sequence of events leading to birth of identical twins is: an egg is fertilized; the fertilized egg (the conceptus) begins to divide into 2, 4, 8, 16, etc. cells; at the blastula or 'ball-like' phase (32 to 128 cells) one of the cells buds off the clump and begins to develop independently. Note that at the blastula phase, one or several cells can be removed from the clump without affecting the outcome of the pregnancy. A fully normal baby is born. In fact, it is now common with in-vitro fertilization to remove one of the blastula cells to check it (destructively) for genetic defects.
The secondary twin is thus the
cloned daughter/son of the primary twin and the granddaughter/grandson of the lady who had all the fun. This twin is
not conceived! She/he is cloned! Those who worry about the prospect of cloning humans should be aware that in 0.4% of all birth events (about 2,000 in the US) human clones are currently produced every year. And those whose religious doctrines teach that the soul enters a human at the moment of conception should worry that 2,000 soulless humans are being born every year in the US alone. The validity of their doctrinal teachings can be easily tested by checking if one of each pair of identical twins is an amoral, antisocial, psychopath.