A woman living on the frontier in the 19th century would be considered an old maid if she was not married and having children by age 16. In our current society, it is almost impossible to function independently before age 30. As a result of this societal transformation, women have increasingly delayed childbearing. Unfortunately, the likelihood of pregnancy declines substantially with age - almost exclusively due to issues with the eggs themselves.
A woman is born with all the eggs that she will ever have. She reaches her full complement of eggs in the ovaries by the time she is about 20 weeks along in her mother's uterus. From that time forward, a certain proportion of eggs move into the maturation process every day. Since there is no cyclic hormonal stimulation by the pituitary gland as of yet, these eggs will reach a certain point of maturity, stop growing, and die off. By the time that woman is born, she will have gone from having about 7 million eggs to having less than 3 million. By puberty, when she has the hormonal capacity to take the eggs to complete maturity, she will be down to only about 300,000 remaining eggs. The good news is that she will only need 300 to 400 in her lifetime, but the bad news is that by the time she is in her mid-30s, the eggs that remain available are, for lack of a better term, the leftovers. The best and most responsive eggs are utilized first.
A major part of egg maturation involves reduction of the number of chromosomes from the immature egg's 92 chromosomes (twice as many as any other cell in the body), down to 23 chromosomes (half as many as any other cell in the body), in order that the sperm's 23 chromosomes will bring the resulting embryo back up to the 46 chromosomes that humans require. Unfortunately, by the time a woman is in her late 30s and early 40s, the machinery that is involved in an even division of those chromosomes has been exposed to enough environmental insults and normal aging that it begins to make more and more errors. This results in embryos that do not have the normal number of chromosomes and do not produce viable pregnancies.
Over the years, multiple approaches have been used to help older eggs wind up with a normal number of chromosomes, but these have been largely unsuccessful. The very low pregnancy rates in women over 40 have led many couples to use donor eggs which, while highly successful, is usually a second-best option.
In recent years, major advances in cell-freezing techniques have allowed us to freeze unfertilized eggs. This has always been a difficult process since the egg, being a large cell and containing a large amount of intracellular water, tends to form ice crystals during the freezing process. Major advances, particularly vitrification, have allowed us to freeze eggs without this complication. The resulting thawed eggs perform nearly as well as fresh eggs, with the exception that intracytoplasmic sperm injection is usually required for reliable fertilization.
The option now exists for women in their 30s to freeze their eggs and thus have a much higher likelihood of chromosomally normal eggs. The process has become commonplace enough that it is no longer considered experimental and has been accepted as mainline therapy for patients. The cost for egg freezing is less than that of a full IVF cycle as the only IVF procedures required are those up to retrieval of eggs and their subsequent cryopreservation. Of course, those further procedures would be necessary when the woman is ready to consider pregnancy - having those eggs thawed and fertilized, then transferring the resulting embryos to the uterus.
Currently, pregnancy rates compare favorably with fresh eggs and are higher than the overall IVF pregnancy rates were only a few years ago. In general, a woman with about 20 eggs frozen prior to the age of 36 has about an 80% chance of delivering a live-born child from this process.