Can a woman of childbearing age with a history of breast cancer take tamoxifen (a selective estrogen receptor modulator) while planning for pregnancy?

Tamoxifen and Pregnancy Planning: Absolute Contraindication

No, you cannot take tamoxifen while planning for pregnancy—tamoxifen is absolutely contraindicated during pregnancy and must be discontinued at least 2-3 months before attempting conception. 1, 2, 3

Why Tamoxifen Must Be Stopped Before Pregnancy

Tamoxifen is a known teratogen with documented fetal malformations and reproductive tract abnormalities, with an estimated risk of fetal complications exceeding 20%. 2 The FDA classifies tamoxifen as Pregnancy Category D, meaning there is positive evidence of human fetal risk. 3

Documented Fetal Risks

• Reproductive tract abnormalities are the most characteristic finding, including ambiguous genitalia in female offspring exposed in utero, similar to DES (diethylstilbestrol) syndrome. 2, 3
• The risk of fetal abnormalities is approximately 12.6% compared to 3.9% in the general population. 4
• Reported complications include vaginal bleeding, spontaneous abortions, birth defects, fetal deaths, and potential long-term risk of vaginal adenosis or clear-cell adenocarcinoma in female offspring. 3
• In a systematic review of 167 pregnancies with known outcomes, 21 were complicated by abnormal fetal development. 4

Required Washout Period Before Conception

You must stop tamoxifen and wait 2-3 months before attempting pregnancy. 1, 3

• This washout period is based on tamoxifen's long half-life and the need to clear the drug from your system. 5
• The FDA label specifically states women should "not become pregnant while taking tamoxifen or within 2 months of discontinuing tamoxifen." 3
• ESMO guidelines recommend continuing active contraception for 3-6 months following the last administered dose. 1

Contraception Requirements While on Tamoxifen

All premenopausal women taking tamoxifen must use effective nonhormonal contraception if sexually active. 3

• Use two methods of barrier or nonhormonal contraception while taking tamoxifen. 2
• Tamoxifen does not cause infertility, even in the presence of menstrual irregularity, so pregnancy can occur. 3
• For women with regular cycles, tamoxifen therapy should be initiated during menstruation. 3
• For women with menstrual irregularity, a negative pregnancy test (β-HCG) is required immediately prior to starting therapy. 3

What If Pregnancy Occurs While on Tamoxifen

If you become pregnant while taking tamoxifen, stop the medication immediately and seek prompt obstetric consultation. 1, 2

• ESMO guidelines state that patients should be informed of the possible increased risk of fetal malformations secondary to first trimester exposure, and pregnancy termination could be considered. 1
• However, the decision should involve informed discussion about the uncertainties and documented risks. 4, 6
• Immediate obstetric referral is necessary for counseling about risks and options. 2

Clinical Management Algorithm

Before Starting Tamoxifen:

• Perform pregnancy test in all premenopausal women 2
• Counsel about absolute need for contraception 3
• Establish two methods of nonhormonal contraception 2

When Planning Pregnancy:

• Stop tamoxifen completely 1, 3
• Wait minimum 2-3 months before attempting conception 1, 3, 5
• Confirm negative pregnancy test before attempting conception 2
• Discuss with oncologist about breast cancer recurrence risk during this interruption 1

If Accidental Pregnancy Occurs:

• Stop tamoxifen immediately 1, 2
• Obtain urgent obstetric and oncology consultation 2
• Perform detailed fetal ultrasound monitoring 2
• Discuss all options including risks of continuing pregnancy 1, 4

Important Caveats

• The duration of tamoxifen interruption for pregnancy may impact breast cancer outcomes, particularly in women with hormone receptor-positive disease who benefit from 5-10 years of endocrine therapy. 1 This risk-benefit discussion must occur with your oncologist.
• Women diagnosed with breast cancer have the lowest rates of subsequent pregnancy among cancer survivors (70% lower than general population), partly due to prolonged tamoxifen treatment periods. 1
• After delivery and completion of breastfeeding, tamoxifen can be resumed to complete the recommended 5-10 year treatment course. 1