Find Out Within the First Trimester Whether or Not Your Child will be Born with Down's Syndrome

Normally, we are born with 23 pairs, or 46 strands, of chromosomes. However, in Down syndrome pregnancies, there is an irregularity present which is referred to as Trisomy 21. This means that the 21^st pair of chromosomes sprouts an extra strand, resulting in 3 strands rather than the usual 2, which puts the total number of chromosome strands at 47. Such an irregularity occurs in the ovum cells during the conception stage, when the chromosome cells themselves are beginning to separate. It tends to affect ovum cells in mature women, for whom the fibers required for cell separation have been stored over a long period of time, leading to reduced flexibility. It is this cause that accounts for up to 95% of all Down syndrome cases.

The next most common cause of the condition is referred to as chromosomal translocation, and this happens when the chromosomes end up in a different location. For instance, the 14^th pair of chromosomes may become lodged against the 21^st pair. Chromosomal translocation is responsible for 4% of Down syndrome pregnancies in younger women.

Least common of all Down syndrome pregnancies is the mosaics condition – whereby both the 46^th and 47^th strands are present in a single fetus – which accounts for the remaining 1% of Down syndrome pregnancies.

Who is at risk of a Down syndrome pregnancy?

• The children of women who fall pregnant after the age of 35 have up to a 1 in 250 chance of being born with Down syndrome. The older the woman is when she becomes pregnant, the higher the risk of a Down syndrome pregnancy.
• Women who have already given birth to a child with Down syndrome have an increased chance of having another child with the condition from any subsequent pregnancies.
• Those with a family history of Down syndrome, for instance in brothers, sisters or other blood related relations, are at risk.
• Women who have undergone an ultrasound examination that produced signifiers associated with Down syndrome are also at risk.

Prenatal Down syndrome screening is a popular screening technique that involves taking a sample of the amniotic fluid during the 16^th-18^th week of a pregnancy. While the technique does provide information as to whether or not there are any fetal abnormalities present, the process of piercing the amniotic sac in order to take the fluid sample means that there is around a 1 in 350 chance of a miscarriage occurring. Obstetricians will therefore only recommend this procedure to women who are viewed as being at high risk, such as those over the age of 35, women who have previously given birth to a child with Down syndrome, women who have returned positive blood screening results, or those who have carried out an ultrasound examination which identified the presence of an abnormality.

An ultrasound examination alongside a blood test can be undertaken from 11-13 weeks onwards. The ultrasound will be able to provide a clear image of the fetus as well as an assessment of the thickness of peripheral lymphatic vessels. In addition, the mother’s blood test can offer signifiers related to PAPP-A and hCG. However, the rate of accuracy is quite low at 80%, while the chances of a false positive result can be as high as 5%.

Down syndrome screening results may be interpreted as follows:

A positive screening result indicates that the mother has up to a 1 in 200 chance of a Down syndrome pregnancy (at the time of taking blood), which means that the chances are as high as they would be in women who fall pregnant after the age of 35. These women will receive medical advice and will be given the option of undergoing an amniotic fluid assessment or removing fetal tissue to be sent away for analysis. Nevertheless, a positive result does not necessarily mean that the infant will definitely be born with Down syndrome, rather that the risk of such an occurrence is higher than in the general population, meaning that an amniotic fluid assessment would be the most appropriate next step.   

A negative screening result means that the risk of a Down syndrome pregnancy is lower than 1 in 200 (at the time of taking blood), which is considered a low risk and that amniotic fluid screening need not take place due to the risk of a miscarriage outweighing the potential benefits of any amniotic screening or fetal tissue removal procedure. That being said, a negative result does not always mean that the child will not be born with Down syndrome, only that the chances of this happening are much lower.

Non-invasive prenatal testing (NIPT) is carried out by taking a sample of the mother’s blood and assessing the cell-free DNA of the fetus which passes into the blood of the mother to identify any irregularities. The procedure offers accuracy rates of up to 99%, while the chances of a false positive result are just 0.02%. If a positive result is returned, amniotic fluid screening must take place to provide a clearer diagnosis. In cases where the screening shows a low risk, the chances of a Down syndrome pregnancy are lower than 1%. This procedure does not pose a miscarriage risk, and can be carried out from the 10^th until the 14^th-18^th weeks of a pregnancy.

The current rates of Down syndrome cases are 1 in every 800 births, which means that there are around 1,000 infants born with the condition in Thailand every year, or around 3 cases per day. Additionally, over 75% of Down syndrome births are either a result of women becoming pregnant after the age of 35, or due to low risk groups not being advised to undergo Down syndrome screening.

Almost all infants with Down syndrome are born into families with no prior history of the condition. Hence, undergoing Down syndrome screening during the early stages, or within the first trimester of a pregnancy, is a hugely important process not to be overlooked.