top of page

Research on the impact of TIMING of insemination within the menstrual cycle 

The probably most often quoted approach to influence the sex of a baby goes back to the studies from Shettles in the 1960s. He suggested that male Y-sperms swim faster, while female X-sperms are more resilient and stay around longer. Based on this assumption, the “Shettles method” suggests having intercourse four to six days before ovulation and then abstaining. 


Even though Shettles findings were quickly disproved, the misinformation around Y and X sperms remain to this day. 


“The widely held idea that spermatozoa bearing the Y chromosome (Y sperm) swim faster than those bearing the X chromosome (X sperm) seems to have originated from Shettles's work in 1960, using phase-contrast microscopy. He claimed to have observed “two distinct populations” of spermatozoa. After attempting to count the chromosomes, he concluded that the smaller heads contain the Y and the larger the X chromosome. There were no intermediate types. The following year he reiterated these findings, adding that smaller headed spermatozoa can migrate more rapidly and fertilise the egg more often in the distal part of the tube.

Reading Shettles's reports in Nature and other peer reviewed journals, many researchers thereafter believed that Y sperm swim faster than X sperm. The finding particularly influenced research on sperm separation.


Although several attempts have been made to correct this impression, it was not until the development of computer assisted sperm analysis (CASA) that reliable observations could be made. So far, researchers have found no morphological differences between human X sperm and Y sperm. Neither mature sperm nor their precursors possess significant morphological differences between X and Y genotypes; and Y bull sperm do not swim faster than X sperm.”

(Grant, 2006)


Shettles findings were discussed broadly and followed by multiple studies – of which most suggested the opposite of what Shettles was proposing: That there is a higher chance of conceiving a girl during the most fertile days of the menstrual cycle – which are the days prior to and during ovulation. 


In 1972 Guerrero examined 1,318 conception cycles and found that “the proportion of male births diminished from 68 per cent six or more days before, to 44 per cent on the day of the shift” (shift being the shift of basal body temperature, indicating ovulation). His work was used by the American scientist Elizabeth Whelan published a book in 1977 (“Boy or Girl”), in which she suggested that intercourse 6-4 days before ovulation will favor boys, while intercourse closer to ovulation will favor girls. These recommendations are often discussed as the Whelan-method. 

Guerreros finding were confirmed by further studies (James 1972, Guerrero, Harlap 1979, France et al 1985, Perez et al 1985). All these studies“show a statistically significant lower proportion of male births among conceptions that occur during the most fertile time of the cycle” (Ronald et al 1991). 


In 1995 a team of scientists around Wilcox studied the timing of intercourse in relation to the sex of the babies of 221 women, of which 129 had a life birth. They concluded in an often quoted article in The New England Journal of Medicine that there is no influence of timing of insemination during the menstrual cycle on the babies gender. 


“Among healthy women trying to conceive, nearly all pregnancies can be attributed to intercourse during a six-day period ending on the day of ovulation. For practical purposes, the timing of sexual intercourse in relation to ovulation has no influence on the sex of the baby.”  (Wilcox et al 1995)


This was confirmed in 1998 by scientists from the ‘The Johns Hopkins University’ in Baltimore 


“Although these findings may be affected by imprecision of the data, the study suggests that manipulation of the timing of insemination during the cycle cannot be used to affect the sex of offspring.”

(Gray et al 1998)


However, reviewing all the evidence found so far, James comes to the conclusion (in 1999, 2000 and 2008), that empirical evidence does exists, that there is a link between timing of intercourse and the gender of the child – with a slightly higher chance of conceiving a girl during the most fertile days of the cycle, shortly before or around ovulation.


Given the amount of research on this topic - pointing to the conclusion that there is either no impact of timing or the opposite of what Shettles was suggesting – it is astonishing how popular Shettles’ theory or method still is. Almost every guide on “How to get a girl” suggests until today that the chance of conceiving a girl increases if you have intercourse four to six days before ovulation and then ‘cut off’ – meaning not having any further intercourse. Given the fact that the chances of falling pregnant are much lower in these early days of fertility, it is no surprise that many parents trying to conceive a girl following the Shettles method end up not falling pregnant at all for many months. 


“The probability of conception ranged from 0.10 when intercourse occurred five days before ovulation to 0.33 when it occurred on the day of ovulation itself. “ (Wilcox et al 1995) 


In conclusion, timing of intercourse seems to have either no or just a small impact on the gender of your child. If anything, there is a higher chance to conceive a girl at your most fertile days, which are around the day of ovulation. 

If you still decide to go with the popular (but opposite) Shettles method, suggesting to cut off intercourse four to six days before ovulation, just be aware that there is no scientific evidence for this and that this can significantly reduce your overall probability of falling pregnant. 

bottom of page