Sex Selection

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X chromosome sperm (female) are larger, slower, and longer-lived than Y chromosome sperm (male). For a female baby, have sex two or three days before ovulation. For a male baby, have sex the day of ovulation. Y sperm are faster swimmers and will reach the ovum before the X sperm. The MicroSort process allows clinicians to separate the larger X sperm from the Y sperm. Pre-implantation Genetic Diagnosis screens embryos for sex during in vitro fertilization.

The one-child policy has certainly contributed to the stark gender imbalance in China, which, according to the 2000 census, was about 117 males to 100 females.  For second births, the national ratio was about 152 to 100.

Bureau of Population, Refugees, and Migration
US Department of State