by 7 days and generally do not correct for leap year. furthermore, actual childbirth has only a certain probability of occurring within the limits of the estimated due date. a study of singleton live births came to the result that childbirth has a standard deviation of 14 days when gestational age is estimated by first trimester ultrasound, and 16 days when estimated directly by last menstrual period. physiology capacity fertility and fecundity are the respective capacities to fertilize and establish a clinical pregnancy and have a live birth. infertility is an impaired ability to establish a clinical pregnancy and sterility is the permanent inability to establish a clinical pregnancy. the capacity for pregnancy depends on the reproductive system, its development and its variation, as well as on the condition of a person. women as well as intersex and transgender people who have a functioning female reproductive system are capable of pregnancy. in some cases, someone might be able to produce fertilizable eggs, but might not have a womb or none that can sufficiently gestate, in which case they might find surrogacy. initiation