What is HCG ?

Human chorionic gonadotropin - or hCG - is frequently called the "pregnancy hormone". Most trying-to-conceive women who have taken more than a few pregnancy tests already know that hCG is the hormone that is detectable by pregnancy test kits (or by hCG blood tests in your doctor's office). However, typically only doctors know the details behind hCG and its function as a reproductive hormone. The purpose of this article is to discuss hCG - what its purpose is, how fast hCG increases in a pregnant woman's system, and when you can begin testing during your cycle.

First of all, hCG is secreted by the placenta shortly following implantation of the egg. To summarize, shortly after ovulation takes place, the egg is fertilized (conception) and a few days later the fertilized egg (now called an embryo) implants in the wall of the uterus. At this point, the hCG hormone starts flowing - produced by the "pregnancy cells" and incipient placental tissues. hCG will pass into a woman's system and will be present in both blood and urine samples.

The Reproductive Function of HCG

Background: Before ovulation takes place, an egg matures in the ovary - in the ovarian follicle specifically. After a woman ovulates and the egg is set free, the reproductive hormones switch gears: estrogen (dominant during the early part of the cycle) yields to progesterone, which is produced by the corpus luteum, a body that develops from the ovarian follicle once the egg is released. Progesterone functions to warm the body and provide a fertile environment during pregnancy. An elevated level of progesterone will also prevent menstruation (causing one of pregnancy's first symptoms - a missed period).

Here, the reproductive function of hCG becomes clear: hCG sends a message to the corpus luteum to keep the progesterone flowing. That's because progesterone supports and nourishes the endometrium lining where the embryo is implanted. If no hCG is detected by the corpus luteum, the levels of progesterone will decrease at the end of the second half of the menstrual cycle and a woman will have her period (and day one of a new cycle begins). However, if conception and implantion take place, and hCG is secreted by the placental tissue, the hCG hormone will trigger the continued release of progesterone by the corpus luteum. In this case, the progesterone level stays high, the body remains warm, menstruation will not take place and the pregnancy will continue! If you happen to be fertility charting, you should note that your bbt temperatures stay high during pregnancy - and do not drop around the date of your expected period. This is called triphasic charting (but's it not the most reliable indicator of becoming pregnant).

Pregnancy Tests and hCG Hormone Levels

As noted on our products page, our pregnancy tests detect the hCG hormone. Because hCG is secreted by developing placental tissue, it's very reliable indicator that a pregnancy has taken place. One of the most common customer support questions we receive is: "When can I begin testing for pregnancy during my cycle?" To understand when, let's take a look at the general timeline of events that take place once ovulation occurs. As noted above, when conception (the meeting of egg and sperm) takes place, the embryo is still "free-floating" in the space of the mother's womb. hCG only starts flowing following implantation. Typically, implantation of the embryo takes place a handful of days after conception - around six days or so on average. Once the embryo implants, hCG is rapidly produced and doubles in early pregnancy every two to three days. Hence, the rate in which hCG increases is very rapid, nearly exponential. This is called hCG doubling time.

With 20 mIU/ml/hCG pregnancy tests (high sensitivity), you can typically begin testing at around seven to ten days past ovulation. On average, this is when the hCG level in a woman's urine starts to exceed the 20mIU/ml/hCG threshold. Do note that "average" does not mean "everybody"! A negative test result this early in pregnancy (7 to 10 DPO) is not necessarily conclusive: hCG develops at different rates among women, and for some women hCG develops slowly. Follow up tests are recommended every 24 to 48 hours using a first morning urine sample. First morning urine will yield the highest volumes of the "pregnancy hormone". Once you determine a positive result on a home pregnancy test, follow up with doctor.... and congratulations!