Which hormone helps maintain pregnancy.

During pregnancy, several hormones play crucial roles in preparing the uterus, supporting fetal development, and preventing premature contractions.

• A) Insulin: This hormone regulates blood sugar levels. While important for maternal health, it is not directly responsible for maintaining the pregnancy itself.
• B) Progesterone: This hormone is often called the "pregnancy hormone." It prepares the uterus for implantation of a fertilized egg and, once pregnancy is established, it maintains the uterine lining, prevents uterine contractions, and supports the developing fetus. It is initially produced by the corpus luteum and later by the placenta.
• C) Thyroxine: This is a thyroid hormone that regulates metabolism. It is important for both maternal and fetal development but is not the primary hormone for maintaining the pregnancy's uterine environment.
• D) Adrenaline: This hormone is involved in the "fight or flight" response and is not directly involved in maintaining pregnancy.

Progesterone is the primary hormone responsible for maintaining the uterine lining and preventing contractions, thereby helping to maintain pregnancy.

The final answer is $\boxed{\text{B}}$.