The vertical dark line on pregnant women's belly, which is termed as pregnancy line is officially known as Linea Nigra. We can even see this line before pregnancy(then called 'Linea Alba', runs from your navel to pubic bone), is usually pale in color, and this line darkens during pregnancy. The pregnancy line actually starts showing up during 5th month of pregnancy, it can even show up early (anytime during second trimester).
As it is a natural part of pregnancy, there are no preventive measures for Linea Nigra. In order to accommodate the growing baby, women's abdominal muscles stretch and slightly separate, thus pregnancy line starts showing up then. This line generally fades away few days/months after delivery. The exact cause of Linea Nigra is still unknown, but it is believed that, hormonal imbalance, and the hormone called 'melanocyte-stimulating hormone'(which is also assumed to cause the darkening around the nipples) is making the line darker during pregnancy.
According to few studies, newborns when placed on mother's abdomen, has the ability to find mother's breast on her own. It is smell, vision and taste that help the newborn to find the breast.
A study named 'Biological mechanisms for homing in on the nipple' states,
When newborn infants are placed in the prone position between their mother's breasts immediately after delivery, they display a consistent sequence of activity, namely, crawling movement, which brings the infants into contact with one of the nipples, and then active sucking, which eventually occurs within about 1 hr after delivery.