How pregnant do you have to be for a blood test to show it? The doctor told me that it takes four weeks, but I read that it is accurate within one week. Which source is correct?

B.

A blood test can detect hCG, the pregnancy hormone, about seven to nine days after fertilization. This is almost a week before you even miss your period. Home pregnancy tests, which analyze the urine, can detect hCG by the first day of a missed period. One home test, called First Response, has recently received approval from the U.S. Food and Drug Administration to state that this test can accurately diagnose pregnancy four days earlier than that!

The confusion you have probably stems from the way we date pregnancies. We date a pregnancy from the first day of your last menstrual period, even though you have not yet conceived at that point. We use this day because most women know when their last period started, but the exact date of conception is much more difficult to pinpoint. Because ovulation and fertilization occur midway through an average 28-day cycle, when doctors say you are "four weeks pregnant," you have actually only been pregnant for two weeks.

Most of the time, unless there is a compelling reason to do otherwise, we do not recommend doing a pregnancy test until after you miss a period. One reason for this is that a large number of conceptions (almost 50 percent) do not implant properly and will miscarry. About half of these occur before a woman even realizes she is pregnant -- so she ends up having a period right around when she was expecting it. There are reasons to do pregnancy tests early. One is if a woman has had a previous tubal pregnancy or multiple miscarriages. Another is if you are engaging in behaviors that could be detrimental to a developing baby; if you know earlier, you may be able to stop the behaviors before they cause harm.