Medical Health Encyclopedia Cycle Days 7 - 13: The endometrium (the inner lining of the uterus) thickens to prepare for the egg implantation.
Ovulation	Cycle Day 14:	Text Continues Below Surge in LH. Largest follicle bursts and releases egg into fallopian tube.
Luteal (Secretory) Phase, also known as the Premenstrual Phase	Cycle Days 15 - 28:	Ruptured follicle develops into corpus luteum, which produces progesterone. Progesterone and estrogen stimulate blanket of blood vessels to prepare for egg implantation.
	If fertilization occurs:	Fertilized egg attaches to blanket of blood vessels that supplies nutrients for the developing placenta. Corpus luteum continues to produce estrogen and progesterone.
	If fertilization does not occur:	Corpus luteum deteriorates. Estrogen and progesterone levels drop. The blood vessel lining sloughs off and menstruation begins.

Click the icon to see an animation about the menstrual cycle.

Stages and Features of Menstruation

Onset of Menstruation (Menarche). Previous evidence had set the onset of menstruation, called the menarche, at an average of age 12 or 13. Recent studies, however, set the time of onset earlier by about one year in Caucasian girls and two years in African American girls. Currently, the youngest possible age for normal puberty is 7 years old for Caucasians and 6 years old for African Americans, down from a previous low of 8 years for both.

Evidence is pointing to the increasing incidence of childhood obesity as a major cause of the trend in earlier menarche onset. (Obesity is also highly associated with hormonal disorders in girls entering puberty at young ages.) Environmental estrogens found in chemicals and pesticides are also suspects.

Length of Monthly Cycle. The menstrual cycle can be very irregular for the first 1 or 2 years, usually being longer than the average of 28 days. The length then generally stabilizes to an average of 28 days, although the cycle length may range from 20 to 45 days and still be considered normal. A variation of 10 days or more--either more or fewer days--may have an impact on fertility, however. When a woman reaches her 40s the cycle lengthens, reaching an average of 31 days by age 49. A number of factors can affect cycle length at any age.