A (fairly) normal uterine cavity and endometrial lining are necessary in order to conceive and maintain a pregnancy. There are several conditions related to the cavity or the lining that can cause problems. One of them is a T-shaped uterus. This is a very rare congenital uterine malformation caused by exposure to DES, a synthetic estrogen compound (drug) that was given to some pregnant women in the 1950's and 1960's. In some cases, the DES exposure caused uterine malformations in the developing female fetus. 

Sometimes these uterine malformations cause problems with a woman's ability to carry a pregnancy to full term. A T-shaped uterus increases the risk for early miscarriages, as well as the risk of preterm labor and delivery. However, some women with a T-shaped uterus carry their pregnancies without problems.

There are also other potential problems that can be caused by DES exposure, such as abnormal Pap smears - and even a predisposition to a type of cervical cancer.