Men product sperm with 22 chromosomes plus either one X (female) or one Y (male) sex chromosomes. Women's ova also have 22 corresponding chromosomes, but they only produce an X sex chromosome. A zygote fertilised by an X sperm grows up to be a girl and a Y sperm results in a boy.

Did You Know?

Nature gives us a balance of about 103 boys to 100 girls.

No method has been developed to influence a baby's sex but by noting the different characteristics of X and Y sperm, it is possible to suggest a way of increasing your chances of conceiving the gender you want:

• X sperm are larger and slower than Y sperm.
• X sperm live longer than Y sperm.
• Intercourse should take place two to three days prior to ovulation for a girl, as only female sperm survive long enough to meet the ovum when it's finally released.
• Intercourse should take place on the day of ovulation for a boy, as the male sperm will reach the ovum more quickly.
• Frequent ejaculation lowers the proportion of Y sperm, so a girl is more likely.
• Infrequent sex increases the proportion of Y sperm and chances of a boy.