The menstrual cycle is the
series of changes your body goes through to prepare for a possible pregnancy.
About once a month, the
uterus grows a new, thickened lining (endometrium)
that can hold a
fertilized egg . When there is no fertilized egg to
start a pregnancy, the uterus then sheds its lining. This is the monthly
menstrual bleeding (also called menstruation or menstrual period) that you have
from your early teen years until your menstrual periods end around age 50
See a picture of a
woman's reproductive system .
The menstrual cycle is measured from
the first day of menstrual bleeding, Day 1, up to Day 1 of your next menstrual
bleeding. Although 28 days is the average cycle length, it is normal to have a cycle that is shorter or longer.
- A teen's cycles may be long (up to 45
days), growing shorter over several years.1
- Between ages 25 and 35, most women's cycles are regular, generally
lasting 21 to 35 days.2
- Around ages 40 to
42, cycles tend to be the shortest and most regular. This is followed by 8 to
10 years of longer, less predictable cycles until menopause.3
Three phases of the menstrual cycle
The phases of
your menstrual cycle are triggered by hormonal changes.
On Day 1 of your cycle, the
thickened lining (endometrium) of the uterus begins to
shed. You know this as menstrual bleeding from the vagina. A normal menstrual
period can last 4 to 6 days.3
your menstrual blood loss happens during the first 3 days. This is also when
you might have cramping pain in your pelvis, legs, and back. Cramps can range
from mild to severe. The cramping is your uterus contracting, helping the
endometrium shed. In general, any
premenstrual symptoms that you've felt before your
period will go away during these first days of your cycle.
During the follicular phase, an
egg follicle on an
ovary gets ready to release an egg. Usually, one egg
is released per cycle. This process can be short or long and plays the biggest
role in how long your cycle is. At the same time, the uterus starts growing a
new endometrium to prepare for pregnancy.
The last 5 days of the
follicular phase, plus ovulation day, are your
fertile window. This is when you are most likely to
become pregnant if you have sex without using
Luteal (premenstrual) phase
This phase starts on
ovulation day, the day the egg is released from the
egg follicle on the ovary. It can happen any time from Day 7 to Day 22 of a
normal menstrual cycle. During ovulation, some women have less than a day of
red spotting or lower pelvic pain or discomfort (mittelschmerz). These signs of
ovulation are normal.
- If the egg is fertilized by sperm and then
implants in (attaches to) the endometrium, a pregnancy begins. (This pregnancy
is dated from Day 1 of this menstrual cycle.)
- If the egg is not
fertilized or does not implant, the endometrium begins to break down.