As women age, the quality and quantity of their eggs start declining. By the time a woman reaches 40, poor-quality eggs usually outnumber healthy eggs. If a poor-quality egg is fertilized, one of two things could happen – either the embryo won’t implant, or it would implant but not develop properly, resulting in a miscarriage.
Though older women (40 and above) are more likely to have bad-quality eggs, younger women below 35 too can have bad-quality eggs. Young women may have poor egg quality due to a number of reasons including immunological and genetic issues. Smoking and cancer treatment can also affect egg quality.