Ans. A. Fertilization of an ovum by an insemination that takes place after one ovum has already been fertilizedConjoined twins (Siamese twins--incidence, 1/50,000) probably result from relatively late monovular separation, as does the presence of two separate embryos in one amniotic sac. The latter condition has a high fatality rate that is due to obstruction of the circulation secondary to intertwining of the umbilical cords. The prognosis for conjoined twins depends on the possibility of surgical separation, which in turn depends on the degree that vital organs are shared. The site of connections varies: thoraco-omphalopagus (28% of conjoined twins), thoracopagus (18%), omphalopagus (10%), craniopagus (6%), and incomplete duplication (10%). Difficult to separate conjoined twins have occasionally survived to adulthood. Most conjoined twins are female.Superfecundation, or fertilization of an ovum by an insemination that takes place after one ovum has already been fertilized, and superfetation, or fertilization and subsequent development of an ovum when a fetus is already present in the uterus, have been proposed as uncommon explanations for differences in size and appearance of certain twins at birth.A prenatal diagnosis of pregnancy with twins is suggested by a uterine size that is greater than that expected for gestational age, auscultation of two fetal hearts, and elevated maternal serum a-fetoprotein or human chorionic gonadotropin levels, and it is confirmed by ultrasound. Ninety per cent of twins are detected before delivery.