Blood test to confirm pregnancy

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Blood test to confirm pregnancy

Blood test to confirm pregnancy

There are different ways to confirm a pregnancy: the urine pregnancy test, available over the counter in pharmacies, drugstores and supermarkets, and the blood pregnancy test performed in the laboratory. Faced with a clinical examination evoking a doubt about the pregnancy or presenting a warning sign, the doctor may prescribe a serum dosage of hCG, which will then be reimbursed.

This reliable test is based on the detection of the hormone hCG in the blood. This “pregnancy hormone” is secreted by the egg as soon as it is implanted, when it attaches to the uterine wall. For 3 months, hCG will keep the corpus luteum active, a small gland which will in turn secrete estrogen and progesterone, essential for the proper development of pregnancy. The level of hCG doubles every 48 hours during the first weeks of pregnancy to reach its maximum around the tenth week of amenorrhea (10 WA or 12 weeks of pregnancy). It then decreases rapidly to reach a plateau between 16 and 32 AWS.

The serum hCG assay provides two indications: the existence of a pregnancy and its good progression according to the quantitative evolution of the level. Schematically:

  • two samples aÌ € a few days apart showing increasing hCG levels testify to a so-called progressive pregnancy.
  • the drop in hCG levels may suggest the end of pregnancy (miscarriage).
  • an uncontrolled progression of hCG levels (doubling, falling, rising) may be a sign of an ectopic pregnancy (GEU). The plasma hCG assay is the basic test for GEU. At the cut-off value of 1 mIU / ml, the non-visualization of an intrauterine sac on ultrasound strongly suggests GEU. Below this threshold, the ultrasound being not very informative, the repetition of the assays after a delay of 500 hours in the same laboratory allows the comparison of the rates. The stagnation or the weak progression of the rate evokes the GEU without however affirming it. However, its normal progression (doubling of the rate at 48 hours) does not eliminate GEU (48).
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On the other hand, the level of hCG does not allow reliable dating of the pregnancy. Only the so-called dating ultrasound (first ultrasound at 12 weeks) allows this to be done. Likewise, while the level of hCG is usually higher in multiple pregnancies, a high level of hCG is not a reliable indicator of the presence of a twin pregnancy (2).

The dosages of the HCG hormone (3)

 

Plasma hCG level

No pregnancy

Less than 5 mIU / ml

First week of pregnancy

An dara seachtain

Tríú seachtain

An ceathrú seachtain

Second and third month

 

An chéad trimester

An dara ráithe

An tríú ráithe

 

10 to 30 mIU/ml

30 to 100 mIU/ml

100 to 1 mIU/ml

 

1 to 000 mIU/ml

from 10 to 000 mIU/ml

from 30 to 000 mIU/ml

from 10 to 000 mIU/ml

from 5 to 000 mIU/ml

 

The blood tests of the first prenatal examination

During the first pregnancy consultation (before 10 weeks), blood tests are obligatorily4 prescribed:

  • determination of blood group and Rhesus (ABO; Rhesus and Kell phenotypes). In the absence of a blood group card, two samples must be taken.
  • the search for Irregular Agglutinins (RAI) in order to detect a possible incompatibility between the future mother and the fetus. If the research is positive, the identification and titration of the antibodies is mandatory.
  • screening for syphilis or TPHA-VDLR. If the test is positive, a penicillin-based treatment will prevent the consequences on the fetus.
  • screening for rubella and toxoplasmosis in the absence of written documents allowing immunity to be taken for granted (5). In the event of negative serology, toxoplasmosis serology will be performed each month of pregnancy. In case of negative rubella serology, serology will be carried out every month until 18 weeks.
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Other blood tests are systematically offered; they are not compulsory but strongly recommended:

  • HIV testing 1 and 2
  • the assay of serum markers (level of PAPP-A protein and hCG hormone) between 8 and 14 weeks. Associated with the patient’s age and the measurement of the nuchal translucency of the fetus at the first pregnancy ultrasound (between 11 and 13 WA + 6 days), this dosage makes it possible to assess the risk of Down’s syndrome. is greater than or equal to 21/1, an amniocentesis or a choriocentesis will be proposed in order to analyze the fetal karyotype. In France, screening for Down’s syndrome is not compulsory. Note that a new screening test for trisomy 250 exists: it analyzes the DNA of the fetus circulating in the maternal blood. The performance of this test is currently being validated with a view to a possible modification of the screening strategy for trisomy 21 (21).

In some cases, other blood tests may be prescribed:

  • screening for anemia in case of risk factors (insufficient food intake, vegetarian or vegan diet)

Intermediate blood tests

Other blood tests will be ordered during pregnancy:

  • testing for the BHs antigen, witness to hepatitis B, in the 6th month of pregnancy
  • a blood count to check for anemia in the 6th month of pregnancy

The pre-anesthesia blood test

Whether or not the mother-to-be plans to give birth under an epidural, the pre-anesthesia consultation is mandatory. In particular, the anesthesiologist will prescribe a blood test to identify possible coagulation problems.

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