If
pregnancy occurs, progesterone is produced
in the placenta, and levels remain elevated throughout the pregnancy. The combination of high estrogen and progesterone levels suppresses further ovulation during pregnancy. Progesterone also encourages the growth of milk-producing glands in the breast during pregnancy.
High progesterone levels are believed to be partly responsible for symptoms of premenstrual syndrome (PMS), such as breast tenderness, feeling bloated, and mood swings.