Womens Health

Pregnancy During and After Taking Oral Contraceptives

Frederick R. Jelovsek MD

"I want to ask you a question concerning something. I am married, and have been for four weeks. I have been taking birth control (although not on time sometimes) but I have been having some suspicion that I am pregnant. I have stopped taking birth control in case this is true, but I was wondering how long I need to be off birth control before I can go get a pregnancy test at the doctor's office. I figure that since the tests tell if you are pregnant by the hormone levels, and since birth control raises the hormone, estrogen in the body, I might need to wait a while before getting a test done so that I can get a truthful answer. I am aware that birth control can give me symptoms that make me think I'm pregnant, but I have a feeling it may not just be birth control. Please give me your opinion as to what I should do if you can ". Mary

Birth control pills, the combined estrogen and progestin pills as opposed to progestin only pills, have a very good record of preventing pregnancy. If a woman takes the pills regularly and is not late or misses any, the pregnancy rate is only 1 per 1000 women (0.1%). No one is perfect however, so the typical use pregnancy rate is higher. Some studies indicate a 3% pregnancy rate and others indicate as high as 5%. Therefore, concern about possibly becoming pregnant while taking pills is common.

Will taking birth control pills make a pregnancy test inaccurate?

No. Pregnancy tests are NOT interfered with by the hormones of oral contraceptives. You are correct that estrogens are elevated in pregnancy and birth control pills contain estrogens, but that is not the hormone that is measured to determine pregnancy. All pregnancy tests measure the beta subunit of a placental hormone called human chorionic gonadotropin (HCG). You will often see it abbreviated as beta HCG. It can be measured in the urine or blood. While it depends upon the sensitivity of the specific test, most home urine pregnancy tests (sensitive to 25 mIU) will turn positive about 13-16 days after conception. If you were not on oral contraceptives, this would mean about the time of the first missed menses. When you are on oral contraceptives, ovulation could have occurred at more irregular times so that all you can say is if the pregnancy test is negative, you are either not pregnant or are less than about 14 days pregnant from ovulation.

How likely is it to get pregnant with missing pills or being late in taking them?

There are many reasons why women may not take their birth control pill on time or even miss them entirely. Such reasons for missing pills include disruptions in their daily routines, their husband's absence, spotting, and just trouble remembering to take them each day. If you are missing pills, you are not alone. One study shows 47% of women missing >=1 pill per cycle and almost a quarter (22%) missing >=2 pills per cycle. Even though this rate of missing pills commonly happens, it is still unlikely to get pregnant. In women off pills for 10 days, only about one in ten will ovulate.

While it is normal to be concerned about possible pregnancy when missing pills, most women can be reassured that they will not get pregnant as long as they do not continue to stay off of the pills.

If I miss some pills in a cycle, should I take them or just stop and wait for my menses?

Ovulation does not usually occur within the first 4 days of missing pills. For many women, however, by the third day of missing pills a menses will usually have started. If you have just missed one pill, take that pill (yesterday's) and today's pill both together and just continue on as per your normal schedule. If you have missed 2-4 pills and just have spotting, you should go ahead and resume taking the pills you are supposed to be on and finish out the pack. Even though the odds are still in your favor not to get pregnant, most doctors would advise you to use back up contraception (condoms/foam) over the next seven days or abstain from further intercourse until that cycle is over.

If your menses has fully started, just stay off of the pills until you have been off for seven days and then start a new birth control pill pack.

What happens if I was taking oral contraceptives when I was already pregnant?

While no one would intentionally take oral contraceptives if they knew they were pregnant, many women each year inadvertently take pills being unaware they are already pregnant. In the U.S. and Europe, in approximately 2-5% of all pregnancies mothers have a history of having taken birth control pills without realizing they were pregnant. Many years ago, with higher dose contraceptive pills, there were concerns that the hormones in pills could produce genital changes in both male and female babies. Subsequent studies have failed to confirm this. At the Hospital for Sick Children in Toronto, Canada, the Motherisk program which tracks various birth defects, there is a good article on oral contraceptives and their effect on pregnancy. This article suggests that there is no need to be concerned about the pills causing birth defects any more than the background rate of birth defects that normally occurs.


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