Health risks of teenage sex include sexually transmitted diseases, inadequately administered contraceptives, and pregnancy. Because a girl's natural immunity is not fully developed, early sexual activity puts her at greater risk from sexually transmitted diseases, reproductive tract infections and cervical cancer. Pregnancy is obviously the most serious risk for girls who are not yet physically mature. The two main obstetric risks of early pregnancy are toxaemia (high blood pressure) and cephalopelvic disproportion (the baby's head is large relative to the size of the mother's pelvis). The baby of a young mother will have a low birth weight either due to premature delivery or to delivery at term of an undernourished foetus. This means the baby is more likely to die at birth or in infancy and that its physical and mental development may be impaired.
Abortion is an option, though it is still illegal in many countries and thus a girl may have to seek an illegal abortion from an unskilled practitioner (complete with all its possible complications). Even in countries where abortion is legal, pregnant teenage girls may not seek such services due to the prior formalities need that may be necessary. In addition, a pregnant girl may not know she is pregnant or may vainly wish it would go away, and thus not seek an abortion until after the first three months of pregnancy, thus increasing the risk of complications.