A Few Weeks Won't Hurt... Or Will It?

While reading about some obstacles in normal fetal development it didn't come to my attention on how serious prematurity could play a factor in cognitive and physical development. My older sister, who is now 24, was born prematurely by about two months. Luckily, she didn't develop any serious cognitive or physical disorders like how many do.
A full-term baby is born after 40 weeks of pregnancy (Lilienfield, 2010). With each week of pregnancy, the odds of disorders decrease and the odds of survival increase (Hoekstra et al., 2004). Research has consistently shown that children born earlier than normal gestation show an increased risk for deficits in learning and other cognitive problems. The Journal of Abnormal Child Psychology published a study to compare the executive functioning of 50 premature children to 50 normal born children (27 boys, 23 girls). They tested children all about around the same age, 6 years old. Each child was tested on the Go/NoGo test, the shape school task, the day-night stroop task, a verbal fluency task, digit span, the object classification task for children, and a full IQ test (Lopez-Duran, 2009). With all these tests, they were able to test all spans of executive functioning. They didn't single out a specific skill such as memory, organizing, speech, etc.
The results show that premature children scored lower on accuracy and efficiency of cognitive switching, verbal fluency, working memory and concept categorization. Although these results show that the children aren't as well developed as normal born 6 year olds, it doesn't necessarily mean that they will have these deficits later in life. As stated in the study, perhaps the children are undergoing a developmental lag and will soon develop the cognitive skills later in their childhood. We can't assume that these will have long-term effects; we have to rule out any rival hypotheses. We also have to take into account how prematurely born the children were and if that perhaps affects the severity of their deficits.
Something that I'm still wondering about on premature babies is what causes them to be born early? Is there a direct cause such as drinking alcohol while pregnant, smoking, or doing any drugs? Are babies more likely to be born prematurely from those actions?