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.