Stretch marks, also known as striae, are due to the stretching of the skin and hormonal changes of pregnancy.
The collagen deep in your skin actually tears and scars as it heals and stretch marks are thin marks, usually no more than a few inches in length, that are often dark red or brown, depending on your skin color. Many women already have some around their thighs from growing during puberty.
More than half of women get stretch marks during pregnancy. They start usually during the second half of the pregnancy. Some often appear on your breasts while nursing.
Pregnancy stretch marks usually appear on the abdomen, but they can also appear on the buttocks, thighs, upper arms, and breasts. If your mother had them, there is a higher chance that you will get them as well.
Gaining a lot of weight will also lead to stretch marks. Trying to gain less weight is probably the best thing you can do to minimize stretch marks. Older pregnant women often get less of them.
Unfortunately, most creams that are meant to prevent them are not very effective. But luckily, the stretch marks often fade after the baby is born and tend to become a little lighter than the rest of the body.