Medicines with teratogenic potential: what is effective contraception and how often is pregnancy testing needed?

New guidance on contraceptive methods and frequency of pregnancy testing to reduce inadvertent exposures during pregnancy in a woman taking a medicine of teratogenic potential.


Some medicines are known or suspected to have the potential to increase the risk of birth defects and development disorders (teratogenic potential) when taken during pregnancy, especially during the first trimester (up to week 12 of pregnancy), when a woman may not know she is pregnant. The product information for these medicines advise that pregnancy should be avoided during treatment, with advice on the need to use contraception including, in some cases, formal pregnancy prevention programmes.

When using any medicine with teratogenic potential, a woman should be advised of the risks and encouraged to use the most effective contraceptive method taking into account her personal circumstances.


Contraceptive methods are designated as effective or highly effective based on their failure rates in typical use in the first year.[footnote 1] ‘Typical use’ includes user error (for example, missed pills, starting a pack late) or use in circumstances that decrease efficacy such as interactions with concomitant medicines.

Highly effective methods have typical-use failure rates of less than 1% and include male or female sterilisation and long-acting reversible contraceptive (LARC) methods (intrauterine devices and implants).

Progestogen-only injections have a typical-use failure rate of 6%, but this may be due to repeat injections being administered late. Progestogen-only injections may be considered as highly effective if repeat injections are documented as having been administered on schedule by a healthcare professional.

Other methods described as effective include combined hormonal contraceptive (pills, patches, or vaginal rings) and progestogen-only pills, which have typical-use failure rates of 9%.

Methods used at time of sexual intercourse or based on fertility awareness have higher typical-use failure rates and are not classed as ‘effective’ for use with medicines with teratogenic potential so should not be relied upon alone.

Need for pregnancy testing

A woman may be unaware she is pregnant at the start of treatment or be in the early stages of pregnancy at the time of repeat prescribing due to contraceptive failure. MHRA continues to receive reports of inadvertent exposure to such medicines during early pregnancy. One way to avoid inadvertent exposures is for a pregnancy test to be performed before prescription of a medicine with teratogenic potential.

Download, print, and use new table

The Medicines for Women’s Health Expert Advisory Group of the Commission on Human Medicines has developed an aide-memoire table to provide guidance to prescribers of medicines with teratogenic potential on the frequency of pregnancy testing needed to avoid exposure in pregnancy during treatment, depending on the chosen contraceptive method.

The aide-memoire table provides a summary of the pregnancy testing advice for the most common contraceptive methods. The table is colour-coded according to the most reliable methods. It is available to download and print, so can be used as a poster in clinics and to update local guidance, as needed.

Download the table now.

Key considerations used in preparing the guidance

The guidance is based on the following considerations:

  • The likelihood of pregnancy is not constant and can vary with changes in a woman’s circumstances during treatment. Therefore, the likelihood of pregnancy should be assessed before each prescription of a medicine with known teratogenic potential

  • Pregnancy tests may not detect an early pregnancy that has occurred after unprotected sex in the preceding 3 weeks. Therefore, women should have a repeat pregnancy test 3 weeks after starting a new contraceptive method if there was any risk of pregnancy at the start of the contraceptive method, even if the first test was negative

  • Modern contraceptive methods have low failure rates (0.03–0.6%) when used reliably and consistently (‘perfect use’), but failure rates are substantially higher for some methods because of user error or interactions with concomitant medicines (‘typical use’). Risk of user error is higher for daily methods than for long-acting reversible contraceptive (LARC) methods and is highest for methods used at time of sexual intercourse

  • Choice of contraceptive method is an individual one and can depend on a number of clinical factors as well as the woman’s personal preference (see current clinical guidance on contraception and statement on teratogenic drugs from the Faculty of Sexual and Reproductive Health [FSRH]). However, different methods have different typical-use failure rates and durations of action, which can affect the frequency of pregnancy testing required

If pregnancy cannot be excluded, the decision to start or continue treatment with a medicine with teratogenic potential will depend on individual circumstances, such as the urgency for treatment and alternative treatment options. If feasible, treatment with a medicine with teratogenic potential should be delayed until pregnancy has been excluded by a repeat test.

Source of guidance

This guidance was produced by the MHRA in consultation with advisory committees the Commission on Human Medicines and Medicines for Women’s Health Expert Advisory Group.

Recommendations are based on FSRH statement on teratogenic drugs and contraceptive failure rates in typical use.[footnote 1]

Article citation: Drug Safety Update volume 12, issue 8: March 2019: 3

  1. Trussell J. Contraceptive failure in the United States. Contraception 2011; 83: 397-404.  2

Published 21 March 2019