Breast development

The rudiments of breasts, including the nipples and underlying structures, are laid down in exactly the same way in both male and females during fetal development.

Typically, the breasts enlarge noticeably during puberty in girls, when female sex hormones (estrogens; also written oestrogens) encourage the sprouting and growth of the underlying mammary gland tissue. Usually this does not occur in males, but the rudiments remain in place.


Digit length

In females, the 2nd digit of each hand (the index finger or pointer) usually extends beyond the 4th digit (the ring finger), whereas in males, the 4th digit is usually longer. The so-called 2D:4D ratio is thought to reflect the level of exposure of the developing skeleton to androgens during fetal life. The ratio in itself has no functional consequences, but may represent a useful readout of fetal androgen exposure or response.


Sex differences appearing at puberty

In boys, increased levels of testosterone produced by the testes at puberty usually induce masculine changes collectively known as virilisation. These changes typically include an increase in size of the penis, testes and scrotum, the appearance of body and facial hair, enlargement of the larynx (the voice box or Adam’s apple) and the resultant deepening of the voice, widening of the shoulders, and the development of body odour. Sperm production commences during this time also.

During puberty in girls, estradiol (also written as oestradiol, a hormone of the estrogen family) causes a comparatively early growth spurt, enlargement of the breasts, widening of the hips, and a tendency for increased body fat to be deposited on the hips, thighs and buttocks. A mix of hormones produced by the hypothalamus, pituitary, and ovaries usually induces the onset of the menstrual cycle.



Last updated: 5 August 2021 PK

Edit history: Author P. Koopman 9/2012; revised PK 5/2013, 9/2013, 7/2015