Services / Gynecology Center / Obstetrics

Why genetic screening is necessary during pregnancy

Pregnancy can be an exciting time in a woman’s life, but it can also be stressful. Many questions, many unknowns, and many unpredictable aspects of one’s pregnancy can worry a patient. Depending on the woman’s age, her individual health concerns, her life circumstances, and her risk factors, there are many elements her gynecologist and her obstetric physician may look at.

Genetic screening is a vital tool for helping diagnose the potential for certain genetic disorders before the patient gives birth. Your physician at the gynecology clinic, who follows you from the start of your pregnancy to about your third trimester, will routinely recommend a variety of screenings, tests and imaging techniques for your peace of mind. These tests provide important information about the health of your baby and may help you maximize your child’s prenatal care and development.


First trimester screening

First trimester screening, also called a combined, two-step test, is a combination of fetal ultrasound and maternal blood tests. This screening process can help determine the risk of the fetus having certain birth defects. First trimester screening is typically done between weeks 11 and 14 of a pregnancy and is a routine prenatal screening test, with no risk of pregnancy complications. Additional testing may sometimes be needed.

Many genetic abnormalities (like Down Syndrome or Trisomy 18) can be diagnosed before birth. First trimester screening is usually optional, and the test results can only show a patient whether they have an increased risk of carrying a baby with chromosomal abnormalities, not whether their baby in fact has one of these conditions.

Other examples of genetic disorders that can be diagnosed before birth are neural tube defects, such as spina bifida.

Second trimester prenatal screening

Second trimester prenatal screening usually includes several blood tests, referred to as multiple markers. These markers provide information about a woman's risk of having a baby with certain genetic conditions or birth defects.

Screening is usually done by taking a sample of the mother's blood between the 15th and 20th weeks of pregnancy. The multiple markers and results can usually indicate a variety of things, such as spina bifida, Down syndrome, other chromosomal abnormalities, defects in the abdominal wall of the fetus, or even the presence of twins or a miscalculated due date.

Prenatal cell-free DNA screening is a blood test that examines fetal DNA in the maternal bloodstream to screen for the increased chance for specific chromosome problems. In addition, it can provide information about the baby's sex. The tests are not 100% accurate, so if an abnormality is detected, additional testing is recommended. Sometimes, a false-positive result may indicate a problem when in fact the baby is healthy.


Testing considerations

Combining first and second trimester screening can provide more accurate results and increases the likelihood that these tests can detect an abnormality. As always, the results are not 100% accurate and you may need to discuss them with your healthcare provider. Sometimes, diagnostic tests, such as chorionic villus sampling and amniocentesis, are recommended for more accurate results, but these tests carry a slight risk of miscarriage.

Genetic testing is optional and, while most are safe, some carry additional risks. There are also benefits in knowing some of this information in advance of the birth. Patients are encouraged to speak with their healthcare providers to make the best decisions for themselves.

Why a dating ultrasound is important?

Now that your pregnancy test came back positive, what’s next? Your next step should be to make an appointment with your doctor, or obstetrician if you already have one, and see them as soon as possible. Your healthcare provider will then most likely do the following: order some blood work, prescribe some prenatal vitamins, and set up a dating ultrasound for you.

What’s a dating ultrasound? It’s ultrasound testing that will provide you (and your doctor) with important information about you and your baby. A dating ultrasound is exactly what it sounds like. It’s basically a test that “dates” your pregnancy with an early ultrasound. Women are often uncertain of exactly when conception happened.

According to the Society of Obstetricians and Gynaecologists of Canada all women should have two ultrasounds: one “dating” ultrasound at 11-14 weeks and one “anatomic” ultrasound between 18-20 weeks.

Why is this information important? A dating ultrasound can tell you how far along you are and how many weeks pregnant you are, based on the size of your fetus. This information, in turn, helps your obstetrician predict the expected date of birth within 5 days, allowing both you and your birthing team to make all the necessary preparations and ensure you get the necessary prenatal care from your first trimester, all the way to your date of birth.


How does a dating ultrasound work and what is it looking for?

Ultrasound uses high-frequency sound waves to create images of the inside of the body. No radiation is used, making it a safe, painless and quick test that usually doesn’t take more than 30 minutes to complete.

The technician applies a warm gel on your abdomen and use a scanning device to get the ultrasound images. Occasionally, the ultrasound must be done through the vagina, a procedure that may be a little more uncomfortable, but not painful.

Dating ultrasounds do more than just “date” the pregnancy and provide the due date for the future mom. They also detect any possible issues and provide a lot of other information about the pregnancy. For example, the number of babies a woman is carrying, the development of her baby, how active and how big the baby is and whether its weight and size are normal for that stage of their development.

They can also detect where the placenta is located, how the baby’s internal organs are developing, any possibility of an ectopic pregnancy or chromosomal abnormalities, and what position the baby is in. This is all important information the obstetrician needs to know. The ultrasound can also take a closer look at the health of the mother’s uterus, ovaries, and the blood flow through the umbilical cord to the placenta.

While seeing your baby for the first time on the black and white screen, and hearing their heartbeat, is an unforgettable experience, the important information gleaned from the ultrasound is ultimately what makes the procedure so important and necessary and a crucial part of a woman’s prenatal care.

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