A pregnancy test becomes positive only after the fertilized egg has implanted into the uterus and sent enough hCG (i.e. the "pregnancy hormone") into your blood stream.
- 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 Pregnancy Test (HPT): The urine pregnancy test will usually not become positive in most women until a missed period or about a week after implantation. The urine test is usually positive at 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 test will be positive 13-14 days after ovulation and fertilization, or about the time you expect your menstrual period.
The amount of time it takes for a pregnancy to be detected and the wide variation of time it takes for a urine pregnancy test to become positive is due to five variables:
- Timing of ovulation
- Timing of implantation
- Blood hCG levels
- A woman's weight (being overweight lowers the hCG)
- Urine hCG level (e.g. how concentrated or diluted the urine is)
- Sensitivity of the pregnancy test
- Timing/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, the day of ovulation is important too, as implantation can only happen if ovulation took place and the egg was fertilized. Therefore, if your day of ovulation was later than you thought, implantation and subsequently the positive pregnancy test will also be delayed. This is why it is really important to keep track of your ovulation day (OD) 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 being filtered through the kidney. After ovulation, the egg gets fertilized and travels through the fallopian tube toward the uterus. This journey takes about a week on average. Implantation, the attachment of the fertilized egg to the lining of the uterus, occurs on average about a week or so 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 a pregnancy can be detected depends on the 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 the 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).
- Blood hCG Level: There is a wide variation of "normal" hCG levels, both in the blood and the urine. This wide range changes the diagnosis of how early a pregnancy can be detected. If a woman has low blood hCG levels, for example, it can take several more days for it to show up on a pregnancy test. If a woman is overweight, with a high BMI, then the blood hCG levels are often lower.
- 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. The time it takes for a pregnancy to be detected depends on the urine concentration. If you drink a lot of fluid, the urine may be too diluted early to achieve an hCG concentration adequate enough for a positive test. Urine pregnancy testing is best done with more concentrated urine.
- Sensitivity of Urine Home Pregnancy Test: Different pregnancy tests have different sensitivities, which affects how soon a pregnancy can 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 amount of hCG to be positive as a test with a 40 mIU/mL sensitivity level; 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.