Knowledge of medical obstetrics has advanced to the point where prospective parents can improve the odds of conceiving the desired gender of baby by following natural methods, as explained in How To Make A Baby Girl Or Baby Boy, the collaborative work of Doctor Mark More and his wife, pediatric nurse Lisa More. When followed properly, the methods are estimated to give 80/20 odds of the desired gender, rather than the usual 50/50. The key to the methods lies in the differences between X-sperm and Y-sperm; X-sperm live longer but Y-sperm are faster, so methods for female conception involved sexual congress a few days before ovulation (giving the Y-sperm time to die off) and making the woman's interior more hostile to sperm, while the methods for male conception are the opposite. The techniques in How To Make A Baby Girl Or Baby Boy do not cost money, or require a doctor's supervision beyond the expenses or necessary medical supervision of any pregnancy. A disclaimer warns that the methods are not guarantees, and further, not all the techniques are recommended for couples who are having a difficult time conceiving; some may even hinder fertility to a degree. The simple instructions and line drawings leave no room for confusion is this useful guide.