There are two types of physical changes that occur during puberty: a) changes to primary sex characteristics and b) changes to secondary sex characteristics. Primary sex characteristics refer to changes to the sexual organs themselves (uterus, vagina, penis, and testes). Secondary sex characteristics refer to other visible changes that mark adult maturation such as changes in height and body shape.
Primary sex characteristic changes for boys include the enlargement of the testes, penis, prostate gland, and seminal vesicles. These changes normally begin to occur between the ages of 9 and 14 years. Their growth is generally completed between ages 12 and 16 years.
The most significant and noticeable puberty-related change for young men is spermarche, or the first ejaculation. Spermarche generally occurs between the ages of 12 and 16. The spermarche indicates a boy is now producing sperm and could fertilize a female egg through sexual activity, resulting in female pregnancy. This first ejaculation often occurs during sleep; it is often referred to as a "wet dream." Young men may also experience spontaneous erections when they're awake. This may occur when they become emotionally stimulated, or for no reason at all. Such spontaneous erections can be uncomfortable or embarrassing for maturing young men. Please refer to the section entitled, "How Parents Can Help Children Cope" for information about helping young men adjust to these physical changes.
The primary sex characteristic changes for girls includes the uterus starting to build a lining that will later be shed through the process of menstruation, and the vagina beginning to produce a mucus-like discharge. This mucus-like discharge is not actual menstruation but rather an early form of vaginal lubrication. As the young woman matures, she can produce this lubrication when she becomes sexually aroused, but she will also produce it in various quantities even when she's not sexually aroused. These changes can happen months or just weeks before a girl experiences menarche, or her first menstrual period. Every youth's experience is slightly different.
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The most significant and noticeable primary sex change for young women is menarche, or the first menstrual period. This occurs when the uterus -- the organ that will later carry and nurture a baby -- sheds its first lining of tissue and blood. In adult women, this lining is shed once a month, or on average, every 28 days. The purpose of the lining is to condition the uterus for the initial portion of pregnancy in which a fertilized egg must attach to the uterine lining. The lining is shed each month when the body realizes it is not pregnant. Women are not "bleeding" when they have their menstrual period; there is no open wound. Instead, their uterus is simply shedding and releasing the bloody lining tissue that has grown inside it over the past month.
Menarche alone does not mean that a girl is able to become pregnant. In some cases pregnancy may occur prior to menarche, while in others it may not be possible for months after menarche. Some girls will not begin releasing eggs from their ovaries for up to 12 to 18 months after they begin their periods. Other girls will have begun releasing mature eggs prior to their first period. For this reason, young women need to be taught that it is possible (even if not common) for them to get pregnant before they have their first period if they should become sexually active.
On average, the first menstrual period occurs between ages 10 and 15 years for North American girls. While some girls may experience a full menstrual flow during their first period, some girls may only experience a light or spotty flow during their first periods. Young ladies may also experience very irregular period intervals and durations in the first months as their cycles regulate. First periods may be uncomfortable or embarrassing for young ladies. Please refer to the section titled, "How Parents Can Help Children Cope" for information about helping young women adjust to these physical changes. A detailed section on the practical knowledge girls need to learn in terms of managing their new monthly cycle may be found here.