It is clear that a variable number of teenage mothers have, to some extent, planned parenthood. From an evolutionary perspective, it could be argued that teenage childbearing may in fact be a positive adaptive mechanism for humans raised in a hostile environment. Life history theory suggests that in risky and uncertain environments the optimal reproductive strategy is to reproduce early in order to maximise the probability of leaving any descendents at all. If some teenagers are planning or hoping for pregnancy, it is possible that some positive behavioural changes may be observed. This has proven to be the case with many teenagers altering their risk taking behaviours in the anticipation and expectation of pregnancy. There is now substantial evidence that teenagers who become pregnant were at higher risk than the general population for using cigarettes, alcohol, and marijuana. However, once pregnant, rates of consumption are usually lower compared with their non-pregnant peers or even their own personal pre-pregnancy rates of consumption.
Quinlivan, J. (2004). Teenagers who plan parenthood. Paper presented at the Australian Society for Psychosocial Obstetrics and Gynaecology Annual Conference. Sydney, NSW, 27-28 August, 2004.