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Sex Development During Pregnancy

Updated: Mar 11, 2021

How baby sex can be define during pregnancy? and how can it be changed?

The sex is determined at the moment of fertilization, but it develops slowly and over a period of several weeks or even months, so doctors are not able to know the sex of the fetus until after a while, by means of ultrasound imaging. So when is the sex of the child determined and how does sex develop during pregnancy?

How is the gender of the child determined?

In humans, each egg released by the mother's ovary contains 23 chromosomes, one of which is the X chromosome (a female chromosome), while the sperm content differs, although all of them carry 23 chromosomes but their content is different, so half of it carries the female X chromosome, while the other half carries the Y chromosome.

When fertilization or fertilization occurs, the egg meets one of these sperms, and the fertilized egg is formed, which contains a complete set of chromosomes (46 chromosomes), among which there are two sex chromosomes, one of which is an X coming from the egg, but the second coming from the sperm will determine the sex of the fetus. If the fertilized egg contains the XY sex chromosomes, it will give a male fetus, but if it contains the XX sex chromosomes, it will give a female fetus.

Thus, the genetic sex of the fetus is determined at the time of conception (fertilization), and the person responsible for determining it is the type of sperm, so doctors rely on the selection of the sperm (sperm screening) in the techniques of sex determination.

How does the development of the sexual organs of the fetus occur during pregnancy?

Each fetus contains opaque structures capable of transforming into male or female reproductive organs, based on genetic factors (chromosomes) and hormonal factors (presence or absence of male hormones).

The development of the external genital organs:

In both sexes, during the fourth week of pregnancy, a small hump called the genital hump develops, and under the influence of the Y chromosome, the testicles secrete male hormones (androgenic hormones), which makes this hump grow and enlarge and turn into a penis.

But in the absence of the Y chromosome, there are no male hormones, so this hump does not grow and remain small and develop to form the female reproductive organs. Although the sex was determined at the moment of fertilization, the growth and development of this hump takes time, so the external genitalia cannot be fully distinguished until about the twelfth week of the fetus’s growth in the womb.

Development of the ovaries and testicles:

In the first weeks of a fetus, the so-called gonads are formed, which are the organs that will develop into testicles that produce sperm or ovaries that produce eggs. Here again, the chromosomes will play an important role in the development of these glands, and the genes on the Y chromosome will direct the fetal gonads to develop into two testes. Later, the testicles, under the influence of male hormones, descend from the abdomen into the scrotum, to settle in it before childbirth, and this is done around the twenty-eighth week of pregnancy or shortly after.

While the female child needs the presence of two copies of the X chromosome for the glands to develop into an ovary, and one copy is not sufficient, and this explains that although the X chromosome in males is present, it is disrupted and cannot push the gonads to develop into an ovary.

What sex disorders can occur?

The development of the sexual organs is a very complex process and is subject to many factors that regulate and control this development, so the diseases and disorders resulting from it are also complex disorders, and they occur when the child's genetic makeup is not compatible with hormones or with the external appearance.

The severity of these disorders ranges from mild problems such as the opening of the urethra in the penis incorrectly, which hinders the process of urination and sexual intercourse after puberty in some cases, and severe cases that lead to the occurrence of what is known as intersexuality (which is the lack of clarity of the genitals). Cases of intersexuality require complex, long and difficult measures, as well as the supervision of a long-experienced team of pediatricians, endocrinologists, radiologists, informants and psychotherapists.

How can the sex of the fetus be revealed?

Parents usually resort to the traditional method of ultrasound, where the doctor can determine the sex at the beginning of the fourth month usually, and this may be delayed a little depending on the child's position and the direction of his back, as the back can block the vision of the genitals sometimes. It is natural that errors occur sometimes, the doctor may expect a female child to discover after a while that she is a male and that the first image is not completely clear, but in general the great development of ultrasound devices that has occurred in recent decades has led to a significant decrease in the proportion of these errors.

The sex of the fetus can also be revealed when carrying out the fetal safety checks (NIPT), and there are other more complex tests that depend on analyzing the genetic material, such as amniotic villi biopsy, but these methods are not resorted to because they are difficult, complex and expensive, and because the sex of the fetus can be detected by other less expensive and complicated means. Resorting to these tests is an option to detect genetic diseases when suspected of their existence.

How can the doctor be involved in determining the sex?

There are dozens of ancient methods that people used to use to determine sex, including the timing of ovulation, the type of food the mother consumes, the Chinese account, and so on, but none of these methods has proven effective. Today, with the development of science and modern technologies, doctors have resorted to other, more accurate and effective methods, such as semen screening to choose the appropriate sperm and injecting the egg into it, or performing genetic analyzes to detect the sex of the fetus before returning it to the uterus while performing tube baby techniques, all of these methods are modern and complex and require extensive expertise and laboratory techniques Sophisticated.


• UNSW Embryology: Genital System Development

• UNSW Embryology: Ultrasound

•University of Michigan Medical School: Reproductive System

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