Pregnancy Test : When does it get positive?

A pregnancy test becomes positive only after the fertilized egg has first implanted into the uterus and then send enough amounts of hCG the pregnancy hormone into your blood stream.

Implantation, when the fertilized egg implants into the endometrium, happens about a week after ovulation (range: 6-12 days), and it takes another 3-4 days after implantation (9-10 dpo: days after ovulation) for the blood pregnancy test to first become positive. 

Blood Pregnancy test: If you are pregnant then the blood pregnancy test is usually positive within 3-4 days after implantation or about 9-10 days after fertilization and ovulation.
Urine (Home HPT) Pregnancy Test: The urine pregnancy test will usually not become positive in most women until a missed period or about a week after implantation. Many manufacturers of urine pregnancy tests want you to believe that the urine test is usually positive before a missed period, but only about 25% of pregnant women will test positive with the HPT 2 days before a missed period, and about 40% the day before the missed period. On average, a pregnancy will be positive 13.5 days after ovulation, or about the time you expect your menstrual period.

The wide variation of when the urine pregnancy test becomes positive and how early can pregnancy be detected are due to four variables which determine when the home urine pregnancy test becomes positive:

1. Timing of Implantation
2. Blood hCG levels
3. Urine hCG level (eg.how concentrated or diluted the urine is)
4. Sensitivity of the pregnancy test

1. Timing of Implantation
A pregnancy test detects the presence of the pregnancy hormone hCG (human chorionic gonadotropin) either in the blood or in the urine. The hCG in the urine comes from the hCG in the blood after it was filtered through the kidney. After ovulation, the egg gets fertilized and travels through the fallopian tube toward the uterus. This travel takes on average about a week. Implantation, the attachment of the fertilized egg in the lining of the uterus, occurs on average about a week or later after ovulation/fertilization, but it can happen as early as six days and as late as 12 days after fertilization/ovulation. At the time of implantation, the placenta starts making the pregnancy hormone hCG, which then enters the blood stream. How early can pregnancy be detected depends onthe pregnancy hormone hcG which can be detected in the blood about three to four days after implantation. Urine hCG can be detected about two to three days after blood hCG can first be seen. Detection of hCG depends on timing of implantation, so hCG can normally be detected in the blood between nine and 16 days after ovulation (HPT: 12-19 days after ovulation).

2. Blood hCG Level
There is a wide normal variation of hCG levels, both in the blood and the urine and this wide range changes the diagnosis of how early can pregnancy be detected. If a woman has low blood hCG levels, for example, then it can take several more days for it to show up on a pregnancy test.

3. Urine hCG Level
Urine hCG levels change over the course of the day depending on how much you drink and how diluted or concentrated the urine is. Urine pregnancy testing is best done with more concentrated urine. So how early can pregnancy be detected depends on the urine concentration. If you drink a lot of fluid, the urine may be too diluted early on in pregnancy to achieve an hCG concentration adequate enough for a positive test.

4. Sensitivity of Urine Home Pregnancy Test
Different pregnancy tests have different sensitivities and that has an impact on how early can pregnancy be detected. The lower the sensitivity, the earlier a pregnancy test becomes positive. Sensitivities are indicated in mIU/mL, the lowest amount of hCG in the urine that tests positive. A sensitivity of 20 mIU/mL requires one-half the hCG level to be positive when compared with a 40 mIU/mL sensitivity level, and it may become positive several days earlier than the pregnancy test with a higher sensitivity. Make sure you read the package of the pregnancy test to determine your pregnancy test's sensitivity.