What are chromosome anomalies?

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When the father’s sperm fertilizes the mother’s egg, the baby receives genetic material from each parent.

 

Every cell in the body contains 46 chromosomes in total, or 23 pairs of chromosomes, that contain all of our genes and DNA. In each pair of chromosomes, one chromosome is inherited from the mother and one chromosome is inherited from the father.

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If a baby has a chromosomal anomaly, this means that there is an abnormality in the baby’s chromosomes that impacts physical or developmental function. The two main causes of chromosomal anomalies are:

 

1. Abnormal number of chromosomes: we normally have 46 chromosomes that form 23 pairs. Anomalies can be due to having too few or too many chromosomes.

2. Abnormal chromosome structure: chromosomes have a very specific physical structure. Anomalies can be due to physical changes in the structure of a chromosome, which includes missing chromosome portions.

 

Chromosomal anomalies impact approximately 1% of all pregnancies, with Down Syndrome being the most common anomaly in Canada, occurring in 1 out of every 800 births.

 

The most common chromosomal anomalies are Down Syndrome (Trisomy 21), Edwards Syndrome (Trisomy 18) and Patau Syndrome (Trisomy 13). In these conditions, the baby has inherited an extra chromosome (referred to as 'tri'somy). Babies with Down Syndrome inherit an extra copy of Chromosome 21, while most babies with Edwards Syndrome and Patau Syndrome have an extra copy of Chromosome 18 and Chromosome 13, respectively.

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What are the components of first trimester screening?

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First trimester screening involves the combining blood tests, ultrasound of the baby and mom’s age to calculate the risk of the baby having a chromosomal abnormality.

 

An important clinical trial, known as the FASTER trial, determined that this is a highly effective strategy for detecting Down’s Syndrome in the first trimester of pregnancy. Testing is typically completed between 11 - 14 weeks of pregnancy and is non-invasive, meaning that testing is not associated with health risks for mom or the baby.

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Here are the components of the test:

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     1. Blood test

Doctors will test mom's blood for levels of two different proteins that are related to the health of the placenta.

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​Pregnancy-associated plasma protein (PAPP-A)

PAPP-A is a protein that is released mainly from the developing placenta and is associated with the baby’s development. In normal pregnancy, PAPP-A levels increase in the mom’s blood throughout pregnancy as the placenta and baby grow. Women with low levels of PAPP-A in the first trimester are at higher risk of having a baby with a chromosomal abnormality. Low levels of PAPP-A are associated with a higher risk of preterm delivery, impaired fetal growth  and pre-eclampsia; however, levels of PAPP-A are typically the lowest in women who have a baby with a chromosomal abnormality.

 

beta human chorionic gonadotropin (β-hCG)

β-hCG is a hormone produced by the placenta that peaks in the first trimester of pregnancy. β-hCG is responsible for maintaining the health of the newly developing placenta and baby until the placenta is able to produce its own hormones. When women take an at-home pregnancy test, this test detects β-hCG in the urine to indicate that a pregnancy exists. Pregnant women with fetal Down’s Syndrome have higher levels of β-hCG, compared to women without chromosomal abnormality.

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     2. Ultrasound of the baby

An ultrasound between 11 and 14 weeks of pregnancy looks for any major physical abnormalities the baby might have. In particular, doctors will check fluid levels that naturally pools at the back of the baby’s neck – this measurement is known as nuchal translucency. Babies with Down’s Syndrome typically have more fluid at the back of the neck, or higher nuchal translucency levels. ​This screening in the first trimester does not assess the presence of neural tube defects.

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Together with mom’s age, maternal blood test levels and the ultrasound of the baby, calculate the risk of the baby having a chromosomal abnormality.

 

This screening is not perfect - if 100 babies had Down Syndrome, first trimester screening would correctly detect approximately 82-87 of these women; 13-18 women would be missed.

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Why does maternal age matter?

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Maternal age above 35 is associated with higher risks of adverse pregnancy outcomes, including miscarriage, gestational diabetes, and chromosomal abnormalities. In Canada, women above 35 are 3 times more likely to have a baby with a chromosomal defect, and women above 40 are 9 times more likely, compared to women aged 20-34 years old. The chances of having a baby with a chromosomal anomaly are:

 

• Women aged 25: 1 in 475

• Women aged 35: 1 in 180

• Women aged 40: 1 in 60

 

Scientists believe that as women age, there are changes to the structure of her eggs located in the ovaries. A study in mice found that the proteins that keep the chromosomes stable inside the egg decrease in level over time. If the chromosomes in the egg are not stable and in correct formation, the division of chromosomes when the egg is fertilized is more likely to be uneven and the chances of the baby having a chromosomal anomaly is higher.

 

This is an important area of research, since maternal ages in Canada and around the world are on the rise. The average Canadian maternal age at first birth in 1920 was 25 years old and is currently over 30 years old. The Canadian Institute for Health Information estimates that 1 in every 5 births in Canada is to a mom over 35 years old.