A pregnancy test, no matter if it's blood or urine, only becomes positive after implantation when the fetus has attached to the uterine wall and connects with the mother's blood system. Implantation, when the fertilized egg implants into the endometrium, happens about 9 days after ovulation and fertilization (range: 6-12 days).
When the pregnancy test first becomes positive has wide variation and depends on several variables. The following variables determine when the home urine pregnancy test becomes positive:
- Exact Day of Ovulation: A pregnancy test depends on the day of implantation, and the urine pregnancy test is usually positive 4-5 days after implantation. However, it's the day of ovulation that's important too, as implantation can only happen if ovulation took place and the egg was fertilized. So if your day of ovulation was later than you thought, then implantation and subsequently the pregnancy test are further delayed too. That is why it's really important to keep track of your OD or ovulation day so you can start counting the days to implantation and eventually the positive pregnancy test.
- 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. hCG can be detected in the blood about 3 to 4 days after implantation. Urine hCG can be detected about 2 to 3 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 9 and 16 days after ovulation (HPT: 12-19 days after ovulation).
- Blood hCG Level: There is a wide normal variation of hCG levels, both in the blood and the urine. 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.
- Urine hCG Level (e.g. how concentrated or diluted the urine is): 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. 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.
- Sensitivity of the Pregnancy Test: Different pregnancy tests have different sensitivities. 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.