The most frequently reported side effects are breast tenderness, headache, nausea, and abdominal pain. You should not use hormone replacement therapy if you are pregnant, because of possible risk to the fetus. Close clinical surveillance of all women taking estrogens is important. Adequate diagnostic measures, including endometrial sampling when indicated, should be undertaken to rule out malignancy in all cases of undiagnosed persistent or recurring abnormal vaginal bleeding. There is no indication for estrogen therapy during pregnancy or during the immediate postpartum period.
Estrogens are ineffective for the prevention or treatment of threatened or habitual abortion.
Estrogens are not indicated for the prevention of postpartum breast engorgement. Estrogen therapy during pregnancy is associated with an increased risk of congenital defects in the reproductive organs of the fetus and possibly other birth defects. The 1985 DES Task Force concluded that use of DES during pregnancy is associated with a subsequent increased risk of breast cancer in the mothers, although a causal relationship remains unproven, and the obeserved level of excess risk is similar to that for a number of other breast cancer risk factors .