Girl or boy: everything is decided during fertilization, at the very moment when the future mother becomes pregnant. Although genetically determined from conception, a baby’s gender is formed in a long process after an undifferentiated phase in early pregnancy. So it takes a little patience for future parents to know the sex of their baby.

The sex of the baby is determined during fertilization, a key moment when the oocyte (or ovum) of the woman and the sperm of the man meet. Each gamete brings its genetic material:

• 22 chromosomes + 1 X sex chromosome on the side of the oocyte;
• 22 chromosomes + 1 X or Y sex chromosome for the spermatozoon.

The two cells merge and form a single cell, the zygote. This very first cell of the embryo contains all the genetic material necessary for the construction of a human being: 44 chromosomes + 1 pair of sex chromosomes. It is this pair that determines the sex of the baby, which depends, in this case, on the sex chromosome provided by the spermatozoon and more precisely on the presence or absence of a Y chromosome.

• the oocyte brings an X chromosome and the spermatozoon an X chromosome: it is a girl (XX);
• the oocyte brings an X chromosome and the spermatozoon a Y chromosome: it is a boy (XY).

Determining the baby’s sex is therefore entirely a matter of chance, depending on the sperm that first succeeded in fertilizing the oocyte. It is therefore impossible to predict the sex of the baby. But one thing is almost certain: there is a one in two chance of having a girl, and a one in two chance of having a boy. The girls / boys birth rates are indeed almost identical: 104,8 boys per 100 girls, according to 2015 figures from INED.

Although the sex of the baby is determined genetically from the moment of fertilization, it remains undifferentiated at the very beginning of pregnancy. The first outline of the gonad, called the genital ridge, appears in the 5th week of pregnancy (7 WA). It is then identical whether it is a girl or a boy. Around the 7th week of pregnancy (9 weeks), this gonad begins to differentiate. Likewise in early pregnancy, all embryos have a tiny prominence between the legs, the genital tubercle. From the 8th week, this tuber also begins to differentiate to form the external genitalia. The differentiation of the external and internal genitalia is complete at the beginning of the 6th month of pregnancy.

In the boy

• at the 7th week of pregnancy (9 weeks), the gonad begins to transform into a testis and the seminiferous cords are formed;
• between the 9th and 14th week, the genital tubercle elongates and forms the outline of the penis. The testes, located at this stage in the posterior wall of the abdomen, already secrete testosterone;
• by the 10th week, the prostate bud is formed;
• During the 7th and 8th month of pregnancy, testicular migration takes place: the testes migrate outside the abdomen to reach the scrotum (the bursae);

In the girl

• around the 8th week of pregnancy (10 weeks) differentiation begins: the central area of ​​the ovary begins to emerge;
• by the 9th week, the utero-vaginal canal, ovaries and fallopian tubes begin to form;
• around the 14th week the genital tubercle, which has only slightly lengthened, regresses to form the clitoris. The slit in the appendix hollows out to form the labia minora and labia majora;
• from the 4th month, the two uterine horns merge into a single uterus, the Müllerian tubercle becomes the cervix and the vagina widens;
• By the 20th week of pregnancy, the ovaries already contain a stock of 6 million eggs. By birth, many will regress.

It is possible under certain conditions to determine the sex of the baby from 12 weeks old, depending on the orientation of the genital tubercle and by taking the measurements of the angles that it forms with the back on the one hand, the abdominal wall of the ‘other. Schematically, in girls, the tuber leaves downwards; in the boy forward.

This early determination is not always possible, however, because sometimes the orientation of the genital tubercle is not obvious (in 7% of cases according to one study (2)). It also includes a margin of error, more frequent when announcing a girl (error rate of 2,6%, against only 0,4% when announcing a boy according to the same study).

The reliability of this method also strongly depends on the experience of the operator, the position of the baby, the term. Also generally the doctors prefer to wait for the second ultrasound of pregnancy (morphological ultrasound of 22 WA) in order to announce the sex of the baby to the future parents, if they wish to know it. However, even at this stage, determining the sex of the baby is not 100% reliable. There is always a small margin of error, depending on the position of the baby in particular.

When there is a risk of a sex-related genetic disease (X-linked recessive diseases, congenital adrenal hyperplasia), it is possible to determine the fetal sex from maternal blood. This method, made possible by the circulation of fetal DNA in the maternal blood, involves targeting sequences of the Y chromosome in the maternal blood. It is a non-invasive alternative to amniocentesis. The reliability of this test is 100% from 8-10 SA (3).