Genetic abnormalities visible during prenatal ultrasound scans

First trimester, Pregnancy

Prenatal ultrasounds are often a moment eagerly awaited by parents-to-be who discover the first images of their baby. Even though most pregnancies result in the birth of a healthy child, the purpose of ultrasound examinations is to detect the presence of birth defects and problems that could have consequences on the baby’s health.

Early ultrasound: Viability and dating

This early ultrasound, which is generally performed between 7 and 11 weeks, is not done for all pregnancies, but can be done in the context of assisted reproduction, if there is a particular history or concern for the pregnancy, or if the gestational age is uncertain.

Its objective is to determine the viability of the pregnancy (presence of the fetal heart), as well as the number of fetuses. In the case of multiple pregnancies, if the pregnancy is far enough along, it can be used to determine whether it is a monozygotic (identical twins) or dizygotic (fraternal twins) pregnancy based on the number of chorionic and amniotic membranes. This also ensures the absence of an ectopic pregnancy and obstetric abnormalities, and a more accurate dating of gestational age than the date of the last menstrual period.

What could be detected during this examination:

This examination can sometimes lead to the identification of a fetus whose heart is no longer beating, or the presence of a blighted ovum or a non-progressive pregnancy. In the case of multiple pregnancies, it is also possible to find what is called a vanishing twin, i.e., out of two embryos initially formed, only one of the fetuses has continued to develop.

There are many causes associated with these lost pregnancies, and it is rare to be able to identify exactly what happened. A large proportion are thought to be due to chromosomal abnormalities which occur accidentally at the time of embryo formation and for which the risk increases with maternal age. For couples who experience multiple miscarriages, genetic testing is available in the biological parents (karyotype) and/or directly on fetal material (product of conception analysis).

First trimester ultrasound: Nuchal translucency measurement

This is often the first ultrasound for most pregnancies with standard follow-up. It takes place between 11 weeks and 4 days and 14 weeks. It can be used to determine gestational age if no dating ultrasound has been performed. It should include measurement of the nuchal translucency and visualization of the nasal bone, which are markers to consider when assessing the risk of chromosomal abnormality, in combination with serum screening or non-invasive prenatal screening. An early evaluation of the fetal anatomy should be performed, as well as assessment of the maternal reproductive anatomy (uterus and associated structures). At this stage, only a non-exhaustive overall assessment of the anatomy of the fetus can be carried out because the development and formation of the organs is not finalized.

What could be detected during this test:

As with the viability ultrasound, sometimes this test leads to the diagnosis of a lost pregnancy. In the case of pregnancies that are still evolving, if the nasal bone is small or not visible, the risk of a chromosomal abnormality (in particular trisomy 21) could be higher, with weighting according to the ethnic origin of the fetus (a small or absent nasal bone would be more common in normal fetuses of Afro-Caribbean, Chinese and Hispanic mothers). It is also possible to identify an increased nuchal translucency (above the standard threshold of 3.5 mm) which may be associated with a higher risk of chromosomal abnormalities or congenital malformations (especially cardiac malformations) in the fetus, or obstetric problems. Even at this stage of development, a good evaluation of the fetal anatomy could identify major congenital anomalies.

The ultrasound signs mentioned above will lead to a referral to a tertiary center for management including genetic counseling. It will then be advisable to discuss with the healthcare professional who follows the pregnancy before proceeding with a screening test (by serum markers or non-invasive prenatal screening) because an invasive prenatal diagnostic test may be recommended to rule out a genetic diagnosis and to help with decision-making for the pregnancy. If abnormalities are confirmed, follow-up ultrasound examinations may be arranged.

Note that ultrasound examinations alone (without the addition of a serum or fetal DNA screening blood test) are not able to detect all cases of trisomy 21.

Second trimester ultrasound: Anatomy scan

This ultrasound can be done between 18 and 24 gestational weeks, but it is ideally done between 20 and 23 weeks. It is the only universal ultrasound in Canada accessible to all pregnant women, and often the last ultrasound before term.

What could be detected during this examination:

Most significant congenital malformations will be identifiable during this anatomy evaluation, as well as severe fetal growth restrictions. Depending on the nature of the abnormality, and whether it is isolated or associated with other findings, the possibility of a genetic or chromosomal syndromic cause can be evaluated by an invasive prenatal diagnosis. Sometimes, certain ultrasound signs called “soft markers” are identified (dilation of the cerebral ventricles, dilated intestinal loops, intracardiac echogenic point, elevated nuchal fold, absent nasal bone, etc.). These signs are not considered pathological, but their presence may require genetic counselling to reassess the risk of trisomy 21 and sometimes lead to a recommendation for invasive prenatal diagnosis or certain tests in the parents to assess the presence of other chromosomal or genetic abnormalities.

Fetal echocardiogram: Fetal heart-specific evaluation

This ultrasound is done from the 22nd week and is only prescribed in certain specific contexts, such as a personal or family history of heart malformations, the detection of an increased nuchal translucency in the first trimester, a suspicion of a fetal heart malformation, or the presence of a biological relationship between the parents (consanguinity).

What could be detected during this test:

The presence of a congenital heart malformation could be detected or confirmed through this evaluation. Depending on the nature of the abnormality, and whether it is isolated or associated with other findings, the possibility of a genetic or chromosomal syndromic cause can be evaluated by an invasive prenatal diagnosis.

Fetal magnetic resonance imaging: a complement to diagnosis

This examination, available from the second trimester onwards, is only prescribed in certain specific contexts, such as the presence of ventriculomegaly or the suspicion of an abnormality, particularly in the central or abdominal nervous system, on ultrasound. This technology provides better contrast overall than fetal ultrasounds and has been shown to be safe for the fetus.

What could be detected during this examination:

The presence of congenital malformations in the central nervous system as well as malformations of the abdominal, urogenital, or intestinal structure could be detected or confirmed through this evaluation. Depending on the nature of the abnormality, and whether it is isolated or associated with other findings, the possibility of a genetic or chromosomal syndromic cause can be evaluated by an invasive prenatal diagnosis.

Note that non-syndromic neurodevelopmental conditions (i.e., those that are not associated with physical signs) including autism and intellectual disability are not detectable during pregnancy.

Third trimester ultrasound: Growth and position of the placenta

This examination, available in the third trimester, is not part of the regular pregnancy monitoring protocol and will generally be prescribed to check a specific anatomical structure or to monitor the baby’s growth curve. It can also be prescribed to verify the position of the baby and placenta.

What could be detected during this examination:

It is very rare but not impossible that certain fetal anomalies not visible during the anatomy scan can be identified during this evaluation. More frequently, this ultrasound could help identify fetal growth restriction, or risk factors for vaginal delivery (breech fetal position, placenta previa or accreta, etc.). If significant concerns are raised due to slowed growth, or a severe risk to the fetus or mother, induction of labour could be decided with the health professional following the pregnancy.

In conclusion

If one of these tests has led to a diagnosis or a strong suspicion of a syndromic genetic condition or congenital malformations requiring special management at birth, it may be recommended to modify the birth plan and the place of delivery to ensure the presence of a specialized neonatal unit, for example in a tertiary centre.

Keep in mind that most pregnancies go smoothly and result in the birth of a healthy child. If you have any worries or concerns about your current pregnancy or future conception, please do not hesitate to talk to your doctor or contact our genetic counselling service to request a consultation and assess your risks.

Claire Bascunana, Genetic Counsellor (MSc, CCGC)

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Genetic abnormalities visible during prenatal ultrasound scans

Par Claire Bascunana Temps de lecture: 6 min
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