Details of genital development
|At conception, gender is determined by chromosome characteristics -|
The genetic sex of a child is established at conception based on the 23rd pair of chromosomes it inherits. The mother's egg contains an X chromosome, and the father's sperm will contain either an X or Y chromosome. It is the male (or rather the male's sperm) that dictates whether the baby will be a boy or a girl.
They have two sets of organs: one that can develop into the female sex organs (Mullerian duct) and one that can develop into the male sex organs (Wolffian ducts).
The gonads will become ovaries or testicles, the phallus will become a clitoris or a penis, and the genital folds will become labia or scrotum. Which sex organs develop depends on the presence of the male hormone testosterone (in humans, the default sex is female).
The SRY gene, on the short arm of the Y chromosome, initiates male sexual differentiation. The SRY influences the undifferentiated gonad to form a testes, which produces the hormonal milieu that results in male sexual differentiation. Testosterone stimulates the Wolffian structures (epididymis, vas deferens, and seminal vesicles), and anti-Mullerian hormone (AMH) suppresses the development of the Mullerian structures (fallopian tubes, uterus, and upper vagina).
Testosterone converts to dihydrotestosterone in the skin of the external genitalia and masculinizes the external genital structures. By 12 weeks most of this male differentiation has occurred, but they are still not completely formed. On ultrasound, if your baby is cooperating, the sex can be identified as early as the 16th to 18th week of your pregnancy. The testicles remain inside the abdomen until late in the third trimester, when they usually descend into the scrotum. Sperm is not produced until puberty.
Female development will occur unless maleness is actively induced by the Y chromosome. If the embryo is female (XX), then no testosterone is made. The Wolffian duct will degrade, and the Mullerian duct will develop into female sex organs. In females, the gonads become ovaries; the uterus, cervix, fallopian tubes, and vagina form; the labia develop; and the phallus becomes a clitoris.
In girls, the ovaries contain over six million eggs, this decreases to approximately one million by birth and will be reduced to about 400 by the time of puberty.
42 days (6 weeks) after fertilization, around week 8 of pregnancy
External genital changes in Embryo development
On ultrasound, if your baby is cooperating and is positioned in a favorable way, his or her sex can be identified as early as the 16th to 18th week of your pregnancy.
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