How
pregnancy tests work
Whether you are having a blood test done in a lab or using
a urine test as home, the both work by detecting the hormone
hCG. There are several types of tests done in the lab that
can detect hCG. Many doctors use the same type of test as
the kits used in home
pregnancy tests. These tests work by using a
substance that changes color once it is bound to hCG. Blood
tests can be used to determine the exact amount of hCG being
produced by the woman whereas urine tests are limited to determining
whether or not hCG is present at a high enough level to confirm
pregnancy. A pregnancy test is considered positive if the
hCG
level is 5 mIU/L or above.
What
is hCG and when will I begin producing it?
To understand how pregnancy tests work it is helpful to know
what hCG is and the basics of early fetal development. HCG,
or human chorionic gonadotropin, is produced very early in
pregnancy and serves several functions to support the developing
baby including stimulating progesterone
and estrogen production.
As
pregnancy begins, your baby goes through many phases of development.
A woman's egg and man's sperm unite on fertilization to form
what is technically now called a zygote. The zygote goes through
rapid division eventually forming an outer layer called a
trophoblast. The trophoblast will begin secreting hCG during
early pregnancy. This does not occur until your baby implants
into the uterus. Ovulation and fertilization typically occur
midway through a woman's menstrual cycle, usually around 14
days after the start of menses. If fertilization occurs, the
zygote will travel down the fallopian tube towards the uterine
cavity. Around seven to nine days after fertilization, your
baby will implant deep into the uterine wall. Your body will
not begin producing hCG until then. Once your body has begun
producing hCG you should be able to determine pregnancy through
a home pregnancy test.
When
should I test?
Newer
home pregnancy tests detect very small amounts of
hCG, some as low as 5 mIU/L. With these very sensitive
tests, it is possible to test positive as early as
seven days past ovulation. It is important to remember
that implantation occurs anywhere from seven to ten
days past ovulation and women produce varying amounts
of hCG, eventually forming a pattern of doubling hCG
amounts every two to three days. While it is possible
to test positive as early as seven days past ovulation,
there is also a good chance that implantation has
not occurred yet or that you are not producing hCG
at a high enough level to be detected by a home pregnancy
test. For best results, it is still recommended to
wait until a woman has missed her period or 14 days
past ovulation. For those that just can't wait, the
very earliest you should test is seven days past ovulation.
