NORMAL OVULATION PHYSIOLOGY
Several hundred to one thousand eggs
which have been dormant since a woman was a fetus in her
mother's uterus become eligible for ovulation
each month. Hormonal changes throughout the menstrual cycle,
as well as local events within the ovary, typically allow
only one egg to be released each month. The egg that is
destined to be ovulated lives inside a water balloon like
structure called a follicle.
The follicle fills with fluid as the egg gets closer to
ovulation. By the seventh day of a woman's menstrual cycle,
a dominant follicle, which will ovulate at mid-cycle is
usually found. The remaining eggs that started the month
with the dominant follicle, but were not selected to ovulate
will be resorbed. As the dominant follicle grows and fills
with fluid, it makes increasing levels of estrogen (estradiol).
Normal ovulation occurs when the dominant follicles at least
20 mm. in greatest dimension, coupled with an estradiol
level ³ 200 pg/ml., and an endometrial lining thickness
of at least 7 mm. Medications such of clomiphene
citrate and injectable FSH/LH allow multiple dominant follicles
to be made each month with the result that several eggs
can be released. In general, each large follicle makes an
estradiol level of approximately 150 to 200 pg/ml.
Ovulation is caused by the brain's release of a luteinizing
hormone (LH). LH triggers ovulation when estrogen levels
have been elevated for between one and two days. The LH
surge coordinates two critical factors necessary for pregnancy
to occur.
1) The physical release of the egg from the dominant follicle.
2) Egg maturation (reactivation of meiosis) that allows
a sperm
to fertilize it.
We frequently use an injection of human
chorionic gonadotropin (hCG) (pregnyl or profasi) to
mimic the LH surge and cause ovulation. Ovulation typically
takes place thirty-eight to forty hours after an hCG injection.
After ovulation the egg lives in the body for only twelve
hours. The end of the fallopian
tube picks up the egg. Fertilization occurs in the ampullary
portion of the fallopian tube. The embryo takes approximately
one week to make its way to the uterine cavity and implant.
There is a very narrow window
of implantation during which time the uterus is receptive
for an embryo to attach and implant. This window spans six
to ten days after ovulation.
After ovulation the dominant follicle produces progesterone
which is critical for maintaining a receptive (velcro) endometrium.
Women undergoing ovulation
induction or in
vitro fertilization will frequently be supplemented
with progesterone in the luteal
phase (the two weeks after ovulation). If pregnancy
is established, the placenta makes hormone calledhCG. hCG
levels rise sharply during early pregnancy and forces the
dominant follicle (now called a corpus
luteum) to make progesterone until the placenta itself
takes over making progesterone at approximately eight weeks
gestation. If pregnancy is not established, the corpus luteum
dissolves and progesterone levels fall. A menstrual period
is then started.
