The Shettles Method is a child conception idea that is reputed to help determine a baby's sex. It was developed by Landrum B. Shettles in the 1960s and was publicized in the book How to Choose the Sex of Your Baby, coauthored by Shettles and David Rorvik. The book was first published in 1971 and has been in print in various editions ever since.
By following the various methods outlined in the book, it is proposed that a couple can affect the probability of having a boy or a girl. Proponents claim between 75 and 90 percent effectiveness, but experts do not agree that the method works; for example, the 1995 article 'Timing of Sexual Intercourse in Relation to Ovulation—Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby', in the New England Journal of Medicine concludes that "for practical purposes, the timing of sexual intercourse in relation to ovulation has no influence on the sex of the baby." However, this stands in stark contrast to an earlier, much larger study from the New England Journal of Medicine (1979) that concluded that "[our] results ... demonstrate that insemination on different days of the menstrual cycle does lead to variations in sex ratio." With such conflicting results, it appears that more studies are needed to determine if the timing of insemination has any impact on the sex of a baby. A 2006 letter  published by BMJ, reviewing some recent research, tries to clarify this issue. It refers that “So far, researchers have found no morphological differences between human X sperm and Y sperm", which seems to dismiss the original claim of Shettles Method.
According to the theory, male (Y) sperm are faster but more fragile than female (X) sperm. Further, acidic environments harm Y sperm, according to the theory, making conception of a girl more likely[page needed]. The Shettles method aims to exploit these two factors.
The Shettles method differs from the Ericsson method, in which the semen is deposited outside the woman and time is given for the fast/slow swimmers to separate before artificial insemination takes place.
Timing of Intercourse
To have a boy, insemination should occur as close as possible to the moment of ovulation so that the faster, Y-sperm arrive first and achieve conception, according to the theory. When seeking a girl, the couple should have sex 2½ to 3 days before ovulation.
Shallow penetration coupled with the sperm deposited close to the entrance favors female conception because the area is more acidic, which inhibits the weaker Y sperm, according to the theory. To allow the Y sperm, which supposedly moves faster, to reach the egg first, use deeper penetration to deposit the sperm at the least acidic area near the uterus opening.
- Wilcox, A J; C R Weinberg; D D Baird (1995-12-07). "Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby". The New England Journal of Medicine 333 (23): 1517–21. doi:10.1056/NEJM199512073332301. PMID 7477165.
- Harlap, Susan (1979-06-28). "Gender of Infants Conceived on Different Days of the Menstrual Cycle". The New England Journal of Medicine 300 (26): 1445–8. doi:10.1056/NEJM197906283002601. PMID 449885.
- Grant, Valerie (2006). "Entrenched misinformation about X and Y sperm". BMJ 332: 916. doi:10.1136/bmj.332.7546.916-b. PMC 1440662. PMID 16613983.
- Hossain, Amjad M.; Sailen Barik; Pandurang M. Kulkarni (January 2001). "Lack of Significant Morphological Differences Between Human X and Y Spermatozoa and Their Precursor Cells (Spermatids) Exposed to Different Prehybridization Treatments". Journal of Andrology 33 (1): 119–123. doi:10.1002/j.1939-4640.2001.tb02161.x. PMID 11191075.
- Shettles, Landrum B.; David M. Rorvik (2006-10-10). How to Choose the Sex of Your Baby: Fully revised and updated (Rev Upd ed.). Broadway. ISBN 0-7679-2610-2.