By Clara Miret Lucio BSc, MSc (embryologist).
Last Update: 04/15/2014

A pregnancy test is one of the most reliable methods to confirm if you are pregnant or not. Usually, a woman starts imagining she could be pregnant when her period does not arrive when it was expected. In addition to this, she also experiences a higher sensitivity in her breasts, headaches, cramps, nausea, etc. After being aware of these symptoms, women decide to take a pregnancy test.

What is a pregnancy

This tests measure the levels of the hCG hormone in blood, which is a key marker of pregnancy. However, this hormone cannot be detected until 14 days after the alleged day of conception. A pregnancy test should be taken by collecting a sample of urine, preferably the first urine after waking up.

High quantities of the hCG hormone begin to be noticed thanks to the embryo, responsible for its increase from the sixth day of pregnancy until the second or third month of gestation, when the placenta starts secreting hormones on its own. After the third month the levels of hCG decrease until its presence in the blood virtually disappears. The main role of this hormone is to maintain the corpus luteum during the beginning of pregnancy. It also intervenes in the immune tolerance during pregnancy, protecting the early fetus from being rejected by the mother’s immune system.

Pregnancy test

Since the presence of this hormone decreases as the pregnancy goes by, a pregnancy test taken during the second or third trimester could be a false negative. This does not mean that the woman is not pregnant, because her belly, the ultrasounds and the other symptoms will prove that she is.

Pregnancy tests in assisted reproduction

In a cycle of assisted reproduction, monitoring the ovulation cycle and working out the precise moment of fertilization is crucial. Before transferring the embryos, doctors run analyses to detect the levels of the hCG hormone 15 days after the ovarian puncture. These analyses are blood analyses, which are more precise than urine analyses. If the concentration is higher than 10 IU/ml, the test can be considered positive. However, when the results are around this threshold, a regular follow-up of the hormonal levels are required in order to rule out a biochemical pregnancy.

False negative pregnancy test

Many women experience the typical symptoms of pregnancy even before confirming it through a test. Some of these symptoms might be psychological or just PMS. Yet, there is a percentage of women who will get false negative results because the test was done too soon or because of other reasons such as:

  • You should check the expiring date of the test or look for visible damages.
  • Make sure that the threshold of the test is 20 International Units of the pregnancy hormone (hCG) per milliliter in blood. If this is not the case, and the test is sensitive to higher thresholds, you might get a false negative. Provided that this happens to you, wait at least one more week so the levels of hCG are higher and can be detected by the pregnancy test.
  • Loss of the embryo. If there was a pregnancy and it came to an early end even when there has not been any bleeding yet, the pregnancy test will be negative.

False negative

Another reason for the test to be negative is that the test is not done at the appropriate time. Is thus, recommended to do the test again about five days after so that the results can be confirmed. Nevertheless, when the results of the pregnancy test are positive, there is no doubt whatsoever. Even though there are false negative test results, there are no false positive results, and if the positive result turns out to be false, it might be because of a tumor or a reaction to the test.

As we have already mentioned, a woman can feel all the pregnancy symptoms and the pregnancy test might still be negative. However, if the symptoms persist, the woman should go to the doctor so he or she can answer all her questions.

Authors and contributors

 Clara Miret Lucio
BSc, MSc
Embryologist
Bachelor's Degree in Pharmacy from the University of Valencia (UV). Master's Degree in Biotechnology of Human Assisted Reproduction from the UV. Experience as a pharmacist and embryologist specialized in Reproductive Medicine at Valencian Infertility Institute (IVI), and currently at Equipo Juana Crespo (Valencia, Spain). More information