How likely is it for someone to get pregnant while on the pill?

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Jane Forester

If you're taking birth control pills, you may wonder what your chances are of becoming pregnant while on the pill. Family Physician Jane Forester has some information for you.

Your question
How likely is it for someone to get pregnant while on the pill? If you're on the pill, will the symptoms of pregnancy seem different? - Erika in New Hampshire

The expert answers
Almost all pregnancies which occur when a woman is on oral contraceptives is from misuse of the pill, unless the pill is well past its expiration date. This is normally from skipping doses or not beginning a new pack on the correct day. Despite these mistakes, the percentage of failure is less than 1 percent of the population.

The main mechanism of action in preventing pregnancy is to prevent ovulation -- which is necessary for pregnancy to occur. Oral contraceptives suppress the follicle-stimulating hormone (FSH) and the luteinizing hormone (LH) from the anterior pituitary. This suppresses the LH surge from occurring and it is this surge which is responsible for ovulation.

Oral contraceptives also alter cervical mucus, making it less penetrable by the sperm. Lastly, it changes the lining of the uterus (endometrium), making implantation less likely. So even if ovulation would occur while on the pill, there are other lines of defense to prevent pregnancy. In general, if one dose is missed, nothing serious will happen, although it is desirable to double the next dose to minimize breakthrough bleeding and stay on schedule. If several doses are missed a barrier technique should be used through the end of the cycle when menstruation should occur. Without any bleeding, the possibility of pregnancy must be considered.

You asked if the symptoms of pregnancy will seem different if a pregnancy is achieved while on the pill. Most obvious will be the lack of menstruation at its expected date. Although, in some cases a light period may occur. However, despite being on the pill, you will experience normal early symptoms of pregnancy such as breast tenderness and nausea. If you suspect that you are pregnant make sure you take a pregnancy test and speak with your physician. Fortunately, there is no harm done to the fetus if there is an overlap of an early pregnancy while taking oral contraceptive pills.

Jane Forester
Family Physician
Glencoe, Illinois

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