Adverse maternal lifestyle habits during pregnancy are important modifable risk factors for pregnancy complications in Western countries. Most common adverse maternal lifestyle habits include smoking, alcohol consumption, and caffeine consumption. Although not directly lifestyle related, maternal age is also considered as a modifable risk factor for adverse pregnancy outcomes. Maternal cardiovascular adaptations might be infuenced by this adverse maternal lifestyle during pregnancy, and  increase  the risks of maternal gestational hypertensive disorders,  including pregnancy-induced hypertension and preeclampsia. Maternal age has also been suggested as risk factor for the development of hypertensive disorders during pregnancy. Maternal cardiovascular adaptations might also be involved in pathways leading to an adverse fetal environment and subsequently neonatal complications. As a result of hypertensive complications  impaired placental perfusion may  occur,  and  subsequently  the  oxygen  and  nutrient  supply  to  the  fetus might  be limited.

Maternal smoking during pregnancy is a well-established risk factor for various adverse pregnancy outcomes, such as fetal death, preterm birth and fetal growth retardation. Women who smoke during pregnancy have offspring with a  lower birth weight of 150 to 200 grams. It has been  suggested  that  the  effects of maternal  smoking during pregnancy on birth outcomes  are trimester specific. High levels of maternal alcohol consumption during pregnancy are associated with birth and long-term developmental defects, fetal alcohol syndrome and increased risks of low birth weight and preterm delivery. Also, previous studies suggested that high caffeine intake  during  pregnancy  is  associated with  increased  risks  of miscarriage  and  fetal  death. Similarly,  previous  studies  suggested  associations  of  higher  levels  of maternal  caffeine  intake during pregnancy with a lower birth weight. Previous studies suggested an inverse U-shaped relationship between maternal age and birth weight. Whether maternal lifestyle habits explain these associations is not known.

Thus, previous studies showed robust evidence for the associations of high exposures levels of maternal smoking, alcohol consumption and caffeine intake with the risks of perinatal mortality and morbidity. However,  less  is known about  the effects of  low  to moderate exposure  levels on these outcomes.  In addition, most previous studies  focussed on birth weight as main outcome measure, but birth weight is just a proxy of fetal growth. Different fetal growth characteristics and body proportions might  result  in  the same birth weight. Exposure  to adverse maternal  lifestyle habits  in different  trimesters of pregnancy might  also have differential  effects on  fetal growth characteristics.  Therefore,  studies  on  exposure  effects  in  different  trimesters  might  identify specifc critical periods. Finally, investigation of factors that may explain the established relationship between maternal age and adverse pregnancy outcomes may help understand the underlying mechanisms. (Figure 1)

Main objectives
The main  objectives  of  the  studies  presented  in  this  thesis  are  to  examine  the  associations  of maternal lifestyle habits with hypertensive complications during pregnancy, and with fetal growth and the risks of neonatal complications.