Whenever we consider a topic like teenage pregnancy it is very easy to allow stereotypical views of teenagers to influence us in our thinking, and so my answer comes with a warning about making generalisations and an encouragement to realise that there are many teenage mothers out there who are excellent parents and look after their children very well.
However, having said this, there are a number of draw backs in terms of having a baby during your teenage years that have been observed in some cases. Firstly, the mother is very young and inexperienced, and, without maturity, can find looking after a baby (which is a big job at the best of times) very demanding. Secondly, often teenagers become pregnant without intending to have a baby, which can result in feelings of anger towards the baby. Thirdly, these feelings of anger or frustration are often compounded by the way in which pregnancy and motherhood necessarily changes the teenager's world. She can see all of her friends carrying on with their lives, graduating from high school and going to college, whereas she must stay at home and put off such plans for a while. Fourthly, some teenagers can consume larger amounts of alcohol and perhaps smoke more than adults, which, if you are pregnant, has obvious health risks. Lastly, though hopefully this is lessening now, having a child so young does bring with it a certain social stigma that can be hard to shake off.