Are you really pregnant? The truth about early pregnancy tests

By Erica Kain

Is it really possible to find out whether I'm pregnant five days before I miss my period? According to the advertisements for one such pregnancy test, finding out early could help you adopt "a healthier lifestyle in the critical first stages of your baby's development."

So why not pay extra for a super-sensitive test and end the suspense? There may be a good reason to wait.

First, a pregnancy test primer: Whether it's a blood test or a urine test done at a doctor's office or in your own bathroom, a pregnancy test is positive when it detects the hormone hCG in your system. The production of even a small amount of hCG means the fertilized egg has been successfully implanted in the uterine lining.

The variance between at-home pregnancy tests is the amount of hCG they can detect, with early pregnancy tests detecting lower levels of the hormone.

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"The commercial pregnancy tests range from 20 to 50 mIU/mL. Since hCG levels double every two to three days, some tests may detect hCG two to three days earlier-so there may be some benefit to using a more sensitive test," says Seth Guller, PhD, the director of the Gyn/Endocrine Laboratory at the Yale School of Medicine.

(Incidentally, the first sign that my last pregnancy was not, as had been diagnosed, a miscarriage, was a blood test at six weeks that indicated a level of more than 44,000 mIU/mL of hCG. That number goes up quickly.)

Guller adds a surprising caveat: Any benefits of early pregnancy detection should be balanced by the fact that the majority of pregnancies ending in miscarriage occur very early.

I hadn't thought of that perspective, but he may have a point. If you've been trying to conceive for a while, why would you want to know of a chemical pregnancy that would only end in the next few days? Wouldn't it be better to test negative and attribute the following bleeding to your period?

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Living well is important while pregnant, but the "healthier lifestyle" that pregnancy test companies promote seems like a marketing tactic. Most women trying to conceive are taking reasonably good care of their bodies in preparation for a potential pregnancy. And, according to Charles Lockwood, MD, the chief of obstetrics and gynecology at Yale-New Haven Hospital, "the risk of exposure to agents causing birth defects doesn't start until two weeks after conception."

But there is no denying that trying to conceive is suspenseful, which is why-despite the emotional risk of learning of brief chemical pregnancies-I always wanted to know if I was pregnant at the earliest moment possible.

However, there are biological factors that may affect the results of these tests. These super-sensitive tests claim to identify a pregnancy a full week before a missed period, but embryos can fully implant (and start producing hCG) at different times. So, even if you're using the most sensitive test, your embryo may not produce a positive pregnancy test right away. In fact, it may take several days. For all of my pregnancies, I tested negative until two days after my period was due.

"Embryo implantation is a process that unfolds over a number of days rather than a single event," says Hugh S. Taylor, MD, the chief of reproductive endocrinology and infertility at the Yale School of Medicine. "The blastocyst generally hatches on day 6 and starts to form a weak attachment very rapidly. The attachment strengthens and then turns into a frank invasion. This process occurs generally between days 6 and 10."

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Our children grow differently, even in the first days following their conception.

"Embryos do grow at different rates; they can vary by about one day prior to implantation," Dr. Taylor says. "Pregnancy tests vary by the growth rate of the embryo and because of maternal body size and hydration status."

So every woman may test positive on a different day, regardless of how fancy her test may be. It might be the first lesson of motherhood: You're running on your child's clock now.