The study, published in the April 30 issue of Human Reproduction, found a fifth of the unplanned pregnancies occurred among women using the birth control pill and 10 percent occurred in women using the intra-uterine device (IUD). Both of those are considered highly effective contraception methods.
One in eight of the unwanted pregnancies were in women choosing condoms and a fifth occurred in women using natural methods, such as male withdrawal or avoiding intercourse on fertile days in the menstrual cycle.
The researchers suggest their findings indicate contraception is still a major problem even in a country such as France, which has the highest use of medical contraception in Europe.
The survey included 1,034 women whose last pregnancy was unintended or who had undergone an abortion and 1,829 randomly selected women in a control group.
Misuse of contraception was the main reason women gave to explain unplanned pregnancies. For example, 60 percent of the birth control pill users said they'd forgotten to take one or more pill.
More than 57 percent of the women whose IUD failed said they didn't know what went wrong or had no explanation, while more than 30 percent said their IUD was in the wrong position or fell out.
In more than half of the unplanned pregnancies that occurred during condom use, the condom slipped off or was torn. In about 30 percent of cases, the women's partners failed to use a condom. About a fifth didn't know what went wrong with the condom or had no explanation.
Here's where you can learn more about contraception.