Any baby born from an encounter: that of an oocyte on the mother’s side and a sperm on the father’s side. Each brings their own genetic material:

• 22 chromosomes + one X chromosome for the oocyte
• 22 chromosomes + an X or Y chromosome for the sperm

Fertilization gives birth to an egg called a zygote, the original cell in which the maternal and paternal chromosomes are united. The genome is then complete: 44 chromosomes and 1 pair of sex chromosomes. From the meeting between the egg and the sperm, all the characteristics of the child are therefore already determined: the color of his eyes, his hair, the shape of his nose, and of course, his sex.

• if the sperm was a carrier of the X chromosome, the baby carries the XX pair: it will be a girl.
• if he carried the Y chromosome, the baby will have the XY pair: it will be a boy.

The sex of the baby therefore depends entirely on chance, depending on which sperm will succeed in fertilizing the oocyte first.

From the 6th week of pregnancy, the primitive sexual cells are put in place where the ovaries or testes will develop later. But even if it is already genetically fixed, at this stage the sex of the fetus remains undifferentiated. In boys, the penis becomes apparent at the 12th week of pregnancy (14 WA – 3rd month), and in girls, the vagina begins to form at the 20th week of pregnancy (22 WA, 5th month) (1). It is therefore at the second pregnancy ultrasound (morphological ultrasound of 22 weeks) that it is possible to know the sex of the baby.

• the Shettles method

According to the work of the American biologist Landrum Brewer Shettles, author of How to Choose the Sex of Your Baby2 (How to choose the sex of your baby), the sperm carrying the female chromosome (X) advance more slowly and live longer, while the sperm carrying the male chromosome (Y) advance faster but survive shorter. The idea is therefore to schedule sexual intercourse according to the desired sex: up to 5 days before ovulation to promote the most resistant spermatozoa in order to have a daughter; on the day of ovulation and the following two days to promote the fastest sperm for a boy. To this are added other tips: pH of the cervical mucus (alkaline with a baking soda vaginal douche for a boy, acidic with a vinegar shower for a girl), depth and axis of penetration, presence of a female orgasm or not, etc. Dr. Shettles reports a 75% success rate… not scientifically proven. In addition, new semen analysis methods have shown no difference in anatomy or speed of movement between X or Y sperm (3).

• the dad method

Based on a study (4) carried out in the 80s at the Port-Royal maternity hospital on 200 pregnant women, this method was developed by Dr François Papa and offered to the general public in a book (5). It is based on a diet providing certain mineral salts in well-defined proportions depending on the desired sex. A diet rich in calcium and magnesium would modify the vaginal pH of the woman, which would block the penetration of Y spermatozoa into the egg, and therefore allow having a daughter. Conversely, a diet rich in sodium and potassium would block the entry of X sperm, optimizing the chances of having a boy. This very strict diet must be started at least 2 and a half months before conception. The author puts forward a success rate of 87%, not scientifically verified.

A study (6) carried out between 2001 and 2006 on 173 women studied the effectiveness of the ionic diet combined with the scheduling of sexual intercourse according to the day of ovulation. Correctly applied and combined, the two methods had an 81% success rate, compared to only 24% if one or both methods were not followed correctly.