First trimester screening – Genetic tests in the first trimester

First trimester screening – Genetic tests in the first trimester

First trimester prenatal screening is a series of procedures performed during the first trimester of pregnancy to determine if the baby may be born with genetic defects. Most of these tests are noninvasive.

Genetic tests indicate possible risks and probabilities of a certain medical abnormality. When the screening test results are abnormal, diagnostic tests can give a definite answer. Women with a high risk of certain genetic conditions are recommended further diagnostic testing

Types of genetic screening

First trimester screening is performed from 10 weeks of pregnancy. They include an ultrasound test and a blood test to examine the fetus’ overall development and test whether the fetus is at risk for genetic disorders such as Down syndrome. The tests can also screen for conditions such as cystic fibrosis and developmental abnormalities.


In an ultrasound test, sound waves are used to create a picture of the fetus. This picture can be used to appropriate the size and location of the fetus, to evaluate the correct development of the pregnancy and locate possible abnormalities in the fetus’ bone and tissue structure.

A special ultrasound examination known as nuchal translucency screening is performed around weeks 11 – 14 of pregnancy. This test measures the accumulation of fluid in the space at the back of a fetus’ neck. Abnormal results indicate a high risk for Down syndrome.

Blood tests

Women undergo two blood tests during the first trimester, to measure the levels of certain substances in the blood. These substances are pregnancy-associated plasma protein and a hormone known as human chorionic gonadotropin. Abnormal levels of these substances indicate an increased risk for chromosomal abnormalities.

During the first prenatal care counselling meeting, blood tests are performed to verify if the mother has received a rubella vaccine, and screen for syphilis. The fetus’ blood type is also determined. Abnormal results indicate a risk to the ability to produce antibodies, which may affect all future pregnancies.

Placenta chorion test

Placenta chorion testing is an invasive screening test in which a small sample is taken from the fetal placenta. The test is performed between weeks 10-12 of pregnancy. The test diagnoses genetic diseases such as Down syndrome and birth defects.

The test can be performed in one of two ways: sampling through the abdominal wall, or through the cervix. There are certain side effects associated with these tests such as cramping, paim=n and a small risk for a miscarriage.

Prenatal diagnostic testing

If a screening test shows abnormal results, the next step is undergoing prenatal diagnostic tests. These tests leave no doubt about certain birth defects.

Diagnostic testing is more invasive than screening tests. It is performed only when there are specific concerns, if screening tests results are abnormal, or if maternal age or family medical history raises a high risk for the genetic defects. Diagnostic testing carries a small risk for miscarriages.

Medical and genetic counselling

Prenatal testing is an important source of information. While most of these tests are routine, some are recommended according to personal considerations. Parents should share their concerns with their doctor and discuss the need for performing the different tests. Parents can also consult genetic counselling. Healthcare providers will discuss the advantages and possible risks of the available tests and help choose the suitable prenatal genetic test.

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