In a study on women who had either been sterilised or were using an intrauterine device (IUD) the frequency of intercourse increased during the six most fertile days of the menstrual cycle and peaked at ovulation despite the fact that these women clearly did not want a baby.
The research team studied 68 sexually active women over three months (a total of 171 ovulatory cycles). The women kept diaries of days when intercourse occurred, and collected daily urine specimens. Researchers later used the samples to identify the fertile days in the cycles. Overall, intercourse was 24% more frequent during the fertile days than during the rest of the cycle.
Lead research Professor Allen Wilcox from the National Institute of Environmental Health Sciences in Durham, North Carolina, said: "There apparently are biological factors promoting intercourse during a woman's six fertile days, whether she wants a baby or not.
"It suggests that couples who 'take a chance' with unprotected intercourse have the deck stacked against them. Intercourse apparently does not happen randomly. It's more likely to occur on the fertile days, even though the average woman won't know when these days are. For whatever reasons (and we don't yet understand the biological reasons behind this) a woman who engages in a single act of unprotected intercourse is more likely to get pregnant than was previously believed."
He said: "It's not uncommon for a doctor to hear from an unhappily pregnant patient that she and her partner had taken a chance 'just this once'. It may be easy to dismiss such claims, but our data suggest these women are probably telling the truth."
Prof. Wilcox and his colleagues from the NIEHS
Contact: Margaret Willson
European Society for Human Reproduction and Embryology