What is Trisomy 18?

Trisomy 18 also called Edwards Syndrome, T18, or Trisomy E and is a genetic disorder, named after John Edwards who first described the disorder in 1960. Around 1 in every 3000 live births are affected by Trisomy 18 and to understand how the disorder occurs, we first need to discuss some basic genetics.

Every living cell in the body contains DNA. This DNA controls what job a cell does, when it divides, when it grows and even what chemical products it makes. DNA is found in lengths and these lengths are called genes. These genes are wrapped up in special structures called chromosomes. The normal number of chromosomes found in a cell is 46, or 23 pairs. Chromosomes are numbered from 1 to 22 and there is one pair of sex chromosomes. When a cell divides, two identical cells are created from one cell (with the same number of chromosomes in each daughter cell).

However, other cells, called sex (or gamete) cells (eggs and sperm cells) only contain 23 chromosomes (half the usual number). This means that when the sperm cell fertilizes the egg, the chromosomes can join together to produce a zygote. This zygote will contain 23 pairs (or 46 chromosomes) and develop into an embryo.

An egg only contains one female sex (X) chromosome, so the sperm cell dictates the sex of the unborn child. The sperm cell may contain another X chromosome (producing a girl) or a Y chromosome that will produce a boy.


What causes Trisomy 18 to occur:

Trisomy 18 occurs when there is some extra genetic material, or even a whole chromosome at chromosome 18 (instead of two chromosomes, there are three). Often the extra chromosome is found in all cells of the infant's body. This is called full Trisomy 18 and occurs in 90% of all cases. Full Trisomy 18 is the most common form of Trisomy 18.

Occasionally, the extra chromosomes are only in some cells of the babies body, and this is called Mosaic Trisomy 18. This form of Trisomy 18 is very rare.

Trisomy 18 usually happens randomly when eggs and sperm cells are formed. The chance of Trisomy 18 increases in older women. Often parents think they are the cause of their child having Trisomy 18, but its essential to know that you have done nothing wrong to cause this disorder to occur with-in your child.


How Trisomy 18 affects the developing fetus:

The developing fetus may have a small placenta that prevents the right amount of materials (such as nutrients) passing from the fetus to the mother. There may only be one umbilical artery (instead of two), preventing adequate circulation between the fetus and the mother.

There is also a risk that there is too little amniotic fluid in the womb. Too little fluid, or oligohydramnios, can prevent the fetus from developing properly. A lack of fluid also causes birth defects and increases the risk of miscarriages or stillbirths. Oligohydramnios may occur because of defects in the developing kidneys.

Too much amniotic fluid, or polyhydramnios usually means that the developing baby is not swallowing and absorbing the usual amount of amniotic fluid. This may be due to problems with the swallowing reflex, or problems within the gastrointestinal (GI) tract (that leads from the mouth to the stomach).

Fetal growth restriction (FGR) may occur when the fetus moves into the womb and fetal growth may even stop. There are also problems with development of the heart, brain and nervous system and many other organs in the body. The fetus may also have weak activity and suffer with distress.


Common medical abnormalities with children born with T18:

  • Spine abnormalities- Spina bifida and scoliosis (curvature of the spine)
  • Congenital heart defects- in about 95% of T18 babies
  • Hearing loss
  • Cleft lip
  • Kidney and urinary tract abnormalities
  • High blood pressure
  • Hernia
  • Clenched hands
  • Rocker bottom feet-(a flat foot malformation)
  • Microcephaly (small head) with back of the head prominent
  • Low set ears
  • Contracted joints
  • Mouth and jaws smaller
  • Feeding problems due to esophageal atresia (esophagus is detached from the stomach)
  • Seizures
  • Delayed growth
  • Learning disabilities



What is the survival rate for Trisomy 18?

Estimates vary on the number of cases of Trisomy 18. During first trimester screening, the incidence is around 1 in 400. It is reported that 1% of known miscarriages are caused through Trisomy 18.

Around 1 in 3000 live births in the US are diagnosed with the condition. Two thirds of these cases are girls.  Around 40% of these infants will survive to one month old and less than 10% of Trisomy 18 babies born will survive after one year old.