Edwards syndrome (Trisomy 18)

00:00 / 00:00



Edwards syndrome (Trisomy 18)


Genetic disorders


Alagille syndrome (NORD)

Familial adenomatous polyposis

Familial hypercholesterolemia

Hereditary spherocytosis

Huntington disease

Li-Fraumeni syndrome

Marfan syndrome

Multiple endocrine neoplasia

Myotonic dystrophy


Polycystic kidney disease

Treacher Collins syndrome

Tuberous sclerosis

von Hippel-Lindau disease




Cystic fibrosis

Friedreich ataxia

Gaucher disease (NORD)

Glycogen storage disease type I

Glycogen storage disease type II (NORD)

Glycogen storage disease type III

Glycogen storage disease type IV

Glycogen storage disease type V


Krabbe disease


Mucopolysaccharide storage disease type 1 (Hurler syndrome) (NORD)

Niemann-Pick disease type C

Niemann-Pick disease types A and B (NORD)

Phenylketonuria (NORD)

Polycystic kidney disease

Primary ciliary dyskinesia

Sickle cell disease (NORD)

Tay-Sachs disease (NORD)

Wilson disease

Cri du chat syndrome

Williams syndrome

Angelman syndrome

Prader-Willi syndrome

Beckwith-Wiedemann syndrome

Mitochondrial myopathy

Klinefelter syndrome

Turner syndrome

Fragile X syndrome

Friedreich ataxia

Huntington disease

Myotonic dystrophy

Down syndrome (Trisomy 21)

Edwards syndrome (Trisomy 18)

Patau syndrome (Trisomy 13)

Alport syndrome

Fragile X syndrome

Fabry disease (NORD)

Glucose-6-phosphate dehydrogenase (G6PD) deficiency


Lesch-Nyhan syndrome

Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)

Muscular dystrophy

Ornithine transcarbamylase deficiency

Wiskott-Aldrich syndrome

X-linked agammaglobulinemia

Autosomal trisomies: Pathology review

Miscellaneous genetic disorders: Pathology review

Muscular dystrophies and mitochondrial myopathies: Pathology review


Edwards syndrome (Trisomy 18)


0 / 11 complete

USMLE® Step 1 questions

0 / 2 complete

High Yield Notes

6 pages


Edwards syndrome (Trisomy 18)

of complete


USMLE® Step 1 style questions USMLE

of complete

A 37-year-old woman, gravida 2, para 1, comes to the office for the evaluation of abnormal laboratory and sonogram results. She previously had a regular menstrual period every 28 days and is currently at 18 weeks gestation. She regularly takes prenatal vitamins and has no other medical conditions. Vital signs are within normal limits. A fetal anomaly scan performed one week ago was significant for polyhydramnios and intrauterine fetal growth restriction (IUGR). A second trimester quadruple screening test is performed at this visit, and the results are shown below:  
 Laboratory value  Result 
 Maternal serum alpha fetoprotein  Low 
 Unconjugated estriol  Low 
 β-HCG  Low 
 Inhibin A  Low 

Which of the following additional findings will likely be seen on the sonogram in this patient?

External References

First Aid








Edwards syndrome p. 61

cataracts and p. 554

chromosome association p. 62

Edwards syndrome (trisomy NaN)


Edwards syndrome p. 61

Trisomy NaN (Edwards syndrome)


Edwards syndrome, named after the British geneticist, John Edwards, who first identified it, is a chromosomal disorder where a person inherits an extra copy of chromosome 18 or a part of it. So instead of having two, they have three chromosomes 18 and so Edwards syndrome is also known as trisomy 18, in other words, “three chromosome 18s”.

All right, our DNA is like this humongous blueprint of information on how to make a human. Usually this massive document is packaged up nicely into a storage bin called a chromosome. Usually we have 46 chromosomes that we use to neatly organize all our information, depending on how you define organize. Each of the 46 chromosomes is actually part of a pair of chromosomes, since you get one from each parent, so 23 pairs. If you wanted to make another human, first you’d have to find someone that feels the same way, and then you both contribute half of your chromosomes, so one from each pair, right? Fifty-fifty. Now, what if someone contributes one too many? Say Dad contributes 23 and Mom contributes 24, is that possible? Yes, and it’s the basis of trisomies, in other words, having three copies of a particular chromosome. Now the most common trisomy in live births is trisomy 21, or Down syndrome. But the second most common trisomy in live births is trisomy 18, or Edwards syndrome.

Alright, so in order to package up half the chromosomes into either a sperm cell or an egg cell, you actually start with a single cell that has 46 chromosomes, let’s just say we’re making an egg cell for the mother, I’m just going to show one pair of chromosomes, but remember that all 23 pairs do this. So the process of meiosis starts, which is what produces our sex cells, and the chromosomes replicate, and so now they’re sort of shaped like an ‘X’—even though there are two copies of DNA here, we still say it’s one chromosome since they’re hooked together in the middle by this thing called a centromere. OK then the cell splits in two, and pulls apart the paired chromosomes, so in each of these cells you’ve now got 23 chromosomes. Now the two copies of the chromosome get pulled apart, and the cells split again, which means four cells, each still with 23 chromosomes. Now these are ready to pair up with a sperm cell from dad that has 23 chromosomes as well, totaling to 46 chromosomes, and voila–nine months down the road you’ve got yourself a baby.


Copyright © 2023 Elsevier, its licensors, and contributors. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Cookies are used by this site.

USMLE® is a joint program of the Federation of State Medical Boards (FSMB) and the National Board of Medical Examiners (NBME). COMLEX-USA® is a registered trademark of The National Board of Osteopathic Medical Examiners, Inc. NCLEX-RN® is a registered trademark of the National Council of State Boards of Nursing, Inc. Test names and other trademarks are the property of the respective trademark holders. None of the trademark holders are endorsed by nor affiliated with Osmosis or this website.