Biologically being male requires one thing – a Y sex chromosome.  

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The Y Chromosome & Testicle Development

Testicles and ovaries develop from the same gonad tissue in a developing baby.  At around the seventh week of pregnancy the SRY gene (which is only found on the Y chromosome) signals the gonad tissue to become a testicle.  Since only the Y chromosome has the SRY gene, it is the Y chromosome that dictates a baby will be a boy.

Once the SRY gene triggers the gonad to become a testicle, the testicle begins to produce three main hormones:

  • Testosterone

  • DHT (dihydroxytestosterone)

  • MIH (short for Mullerian Inhibiting Hormone)


These hormones cause the creation and development of the male reproductive system and the female reproductive tracts to regress.


Reproductive Tract Development

When a baby is first conceived, it has two separate duct systems.  The Wolffian duct can develop into the male reproductive tract while the Mullerian duct can develop into the female reproductive tract. 

Testosterone and DHT stimulate the male duct (Wolffian duct) to become the epididymis, vas deferens, ampulla, and seminal vesicles.  At the same time, the testicles secrete MIH which actually “kills” the Mullerian duct that would have become the female tract.  All that remains of the Mullerian tract in males is a small opening/pouch in the prostate called the prostatic utricle. 

If there were no testosterone to stimulate the male duct and if there were no MIH to kill the female duct, the Mullerian tract would form into the fallopian tubes, uterus, and most of the vagina and the male ducts would wither away.   See the figure to the below:

Reproductive Tract Development Updated 9

Genitals & Sexual Gland Development


All babies have tissues that will grow into different genitals and reproductive glands depending on whether or not they are stimulated by testosterone and, most notably, DHT (dihydroxytestosterone).  For example:

  • Erectile tissues elongate and fuse into a penis when stimulated by testosterone/DHT

    • In girls, the lack of testosterone/DHT causes the erectile tissues to remain separate with one cavernosa inside each labia and the spongiosum becoming the small clitoris

  • Protective skin in the groin fuses together to form the scrotum when stimulated by testosterone/DHT

    • In girls, the lack of testosterone prevents the skin folds from fusing, thus becoming the external labia majora

  • “Ejaculation Gland” tissue is stimulated by DHT to swell and enlarge into the prostate gland

    • In girls, the lack of DHT causes this tissue to remain small creating the lesser vestibular (Skene) glands

  • “Lubricating Glands” are stimulated by DHT to form into the bulbourethral (Cowper’s) glands and periurethral glands (Glands of Littre) that drain into the penis to form pre-ejaculate

    • In girls, the lack of DHT causes this tissue to form into the greater vestibular (Bartholini) glands to lubricate the vagina


As you can see, nearly all of these embryonic structures have corresponding functions in men and women

  • Penis and clitoris both involved in rigidity and sexual stimulation

  • Scrotum and labia both act as a protection to the reproductive tract and/or organs

  • Prostate and Skene glands involved in releasing fluid during orgasm

  • Bulbourethral and Bartholini glands both involved in secreting lubricant-like fluid during sexual stimulation


In summary, the SRY gene on the Y chromosome dictates a testicle to be formed.  But it’s the hormones released from the testicles that are responsible for the growth of the male reproductive system. 


At six weeks (see below) the male and female genitals look nearly the same.  This is because the testosterone and DHT have not yet stimulated the genital tissues to start elongating into a penis and fusing into a scrotum.  This is a main reason why ultrasounds performed during the first trimester often can’t determine if the baby is a boy or a girl (ultrasounds are looking for the presence or absence of a penis to identify the gender).  Newer methods involving lab tests can now determine the gender of a growing baby earlier.

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Below is shown a fetus at 10-12 weeks (toward the end of the first trimester).  At this point, the penis starts elongating and the urethra in the male begins the process of forming into a long tube and closing (see image below).  

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If testosterone/DHT is not maintained throughout a pregnancy with a male fetus, it is possible for the urethral opening to remain open.  This can lead to baby boys being born with the urethra exiting either lower down the penis, in the scrotal area, or even behind the scrotum.  This occurs most often on the underside of the penis and is called a hypospadias.  It occurs in about 1 in 200 male births and can be corrected soon after birth.  In other cases the urethra may form an open groove or trough on the back side of the penis.  This is rarer and is called an epispadia (click HERE to learn more).  Many babies that are born with urethral abnormalities can also have a penis that curves.  However, the penis can curve during development for other reasons.  As the penis elongates during fetal development, it isn’t uncommon for one column of erectile tissue to grow faster than the others.  Many baby boys have a bent or curved penis during development.  By the end of birth, the other columns of erectile tissue have caught up with one another; however, if there is a disruption in testosterone or DHT, the process of penile elongation can halt at a curvature stage and a baby can be born with a curved penis (click HERE to learn more).

Near birth, the penis is now formed and straight and the scrotal tissue has fully fused together forming a sac (see below).  Now that the scrotum is fully formed, the testicles are then able to drop from their position inside the boy’s body. 

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Testosterone exposure is so important to a developing baby boy.   If the mother is exposed to testosterone blockers, the boy’s genitals may physically appear more female than male.  In some cases, lack of testosterone and DHT can cause the growing baby boy to die.  Because of this, women should check with their healthcare provider to make sure the medications they’re taking are safe during pregnancy.  In addition, men should be aware that the medication they take before and during their wife’s pregnancy may cause birth defects as well.  On the other hand, a developing baby girl should NOT be exposed to testosterone so her genitals will develop normally.  For this reason, pregnant women should not touch testosterone gels; and men who use testosterone gels should make sure they do not rub those areas against pregnant women.  


The following diagram shows where certain tissues are located and how they develop into the male (left) or female (right) genitals based on exposure to testosterone.

Pre-Differentiation (6 weeks)

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Differentiation Phase (10-12 weeks)

12 week male.jpg
12 week female.jpg


Birth male.jpg
birth female.jpg

Testicular Descent


During pregnancy, the testicles form inside the boy’s abdomen (much like ovaries in girls).  At around three months of pregnancy, a canal forms between the scrotum and the abdomen – this is called the inguinal canal.  Over time the testicle will descend through this canal into the scrotum.  The gubernaculum is a ligament (dense band of tissue) that connects the bottom of the testicle on one end and the base of the scrotum on the other.  This tissue shortens over the pregnancy, thus pulling the testicle down through the inguinal canal.[i]  Usually around the  ninth month (shortly before birth) the testicles finally descend fully into the scrotum. 

Sometimes the testicles do not fully descend before birth or the testicle reverses back into the inguinal canal after birth (click HERE to learn more).


After a testicle has descended, the gubernaculum anchors the testicle to the scrotum preventing it from twisting and turning in the scrotum.  A weak anchoring of the gubernaculum can cause the testicle to twist and kink the blood supply to the testicle leading to testicular torsion (click HERE to learn more).

The opening of the inguinal canal results in hernias which can be more common in babies than in older men.  Inguinal hernias occur when the intestines can also pass through this canal into the scrotum (click HERE to learn more).

Testicles in Abdomen

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Testicles Begin Descent

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Testicles Enter Inguinal Canal

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Testicles Exit Inguinal Canal & Enter Upper Scrotum

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Testicles Complete Descent & Adhere to Base of Scrotum

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All illustrations on this page are original design