Pregnancy and Pregnancy Testing: FAQs

Clinic A/P, adults, OB/GYN

What is the lifespan of an egg and a sperm?

  • The egg last 12-24 hours (about 1 day)
  • The sperm lasts up to 5 days inside the woman’s body.

“Normal sperm can survive in the female reproductive tract and retain the ability to fertilize an egg for at least 3 and up to 5 days, but an oocyte can be fertilized successfully for only approximately 12–24 hours after ovulation. Consequently, virtually all pregnancies result from intercourse occurring sometime within the 6-day interval ending on the day of ovulation.” (Fritz and Speroff)

What are the minimum hCG levels required to get a positive urine and serum pregnancy tests?

Urine pregnancy test Serum pregnancy test
The minimum hCG level needed for a positive test Qualitative test: 20 to 50 milli-int. units/mL, depending on test Qualitative test: 5 to 10 milli-int. units/mL, depending on the test

Quantitative test: 1 to 2 milli-int. units/mL for an ultrasensitive test

Urine pregnancy tests may only detect total hCG levels ≥20 mIU/mL. For serum tests, the threshold for a negative result varies (ie, <1 versus <5 mIU/mL).

What is the earliest time you can test for pregnancy?

All pregnancy tests are based on detecting hCG in the blood or urine. “HCG is secreted into the maternal circulation after implantation, which occurs 6 to 12 days after ovulation.  This is the earliest that hCG can be detected with an ultrasensitive test.” 

Ovulation generally occurs around day 14 of a 28-day menstrual cycle. Implantation occurs about 6 to 12 days after ovulation. For a 28 day cycle, if ovulation occurs on day 14, implantation would happen around day 20 to 26 of the cycle. It’s only after implantation that hCG is secreted.

“For a pregnancy to occur, the conceptus (a term for the product of fertilization) must implant in the uterus. Implantation occurs about six to seven days after fertilization when the conceptus arrives at the uterus.” Washington U.

When should a urine pregnancy test be taken?

Urine pregnancy tests qualitatively test for the beta subunit of hCG and are usually positive within one week of missed menses (AAFP 2014).
“Almost all pregnant women will have a positive urine pregnancy test one week after the first day of a missed menstrual period.” UTD

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hCG is made by trophoblast, a layer of tissue on the outside of the blastocyst that provides the embryo with nutrients and later forms part of the placenta and the fetal membranes. Lab tests for hCG are almost 100 percent sensitive and specific for the diagnosis of the trophoblast-related conditions (pregnancy and the gestational trophoblastic diseases).

“hCG is made by cells formed in the placenta, which nourishes the egg after it has been fertilized and becomes attached to the uterine wall.  Levels can first be detected by a blood test about 11 days after conception and about 12-14 days after conception by a urine test. Typically, the hCG levels will double every 72 hours. The level will reach its peak in the first 8-11 weeks of pregnancy and then will decline and level off for the remainder of the pregnancy.” American Pregnancy.

What Causes False-negative Pregnancy Tests?

-The test is done too early after conception. As such, the hCG concentration is below the threshold required for a positive test
-The hCG isoform measured is different from the hCG isoform in the sample. This pertains mostly to urine pregnancy tests
-Hook effect due to extremely high hCG concentration (>500,000 milli-int. units/mL, these levels are most commonly seen in gestational trophoblastic neoplasia)

What Causes False Positive Pregnancy Tests?

  • “Recent first-trimester pregnancy loss (induced or spontaneous) in which hCG levels, though declining, are still elevated.
  • Pregnancy loss very soon after implantation
  • hCG secretion from a tumor
  • Pituitary hCG secretion
  • Interference from human antibodies against animal antibodies or heterophilic antibodies (serum test positive but urine hCG will be negative)
  • Patient has received a medication containing hCG or certain antibodies”

 

“Embryo is the term used to describe the developing human being up until 8 weeks, after which time the term fetus is used.”

References

  • Clinical Gynecologic Endocrinology and Infertility by Marc Fritz, MD, Leon Speroff, MD
  • https://courses.washington.edu/conj/bess/implantation/implantation.htm
  • http://www.aafp.org/afp/2014/0201/p199.html
  • https://www.uptodate.com/contents/clinical-manifestations-and-diagnosis-of-early-pregnancy
  • https://www.uptodate.com/contents/human-chorionic-gonadotropin-testing-in-pregnancy-and-gestational-trophoblastic-disease-and-causes-of-low-persistent-levels
  • http://americanpregnancy.org/while-pregnant/hcg-levels/

 

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