- Babies with trisomy 18 appear thin and frail. They fail to thrive and have problems feeding. Trisomy 18 causes a small head size, with the back of the head (occiput) prominent. Ears are usually low set on the head. The mouth and jaw are unusually small, and there is a shortened sternum (breastbone). 

- At birth, these babies are small for their age, even when delivered full-term, and have a weak cry. Their response to sound is decreased and there is often a history of infrequent fetal activity during the pregnancy. About 90 percent of babies with trisomy 18 have heart defects. They clench their fists in a characteristic manner and extending the fingers fully is difficult. Joint contractures - where the arms and legs are in a bent position, rather than relaxed - are usually present. The feet may be referred to as "rocker bottom" due to their shape.

- Babies with trisomy 18 may also have spina bifida, eye problems, cleft lip and palate, and hearing loss. It is also common to see feeding problems, slow growth, seizures, high blood pressure, kidney problems and scoliosis (curvature of the spine). In males, the testes sometimes fail to descend into the scrotum. 

- Most babies with trisomy 18 have problems which affect all parts of the body in some way. Most children with trisomy 18 will have some, but not all, of the health problems mentioned here. Heart problems, feeding difficulties, and an increased susceptibility to infection are factors which, most often, contribute to the death of these children.

- Babies with trisomy 13 often have a low birthweight, even when born full-term. They have a small head, with a sloping forehead. Usually, there are major structural problems with the brain which are diagnosed shortly after birth. Often, the front of the brain does not divide properly, resulting in a condition called holoprosencephaly. This can cause changes in the development of the baby's face, where the eyes are close set, or the nose or nostrils are underdeveloped. Cleft lip and cleft palate are present in 60 to 80 percent of babies with trisomy 13.

- Eye problems are common and the ears are low-set and unusual in shape. Sometimes, babies with trisomy 13 can have scalp abnormalities (cutis aplasia) which resemble ulcers. They can also have birthmarks which are purplish-red in color; the color is due to tiny blood vessels close to the skin (hemangiomas). 

- Many babies with trisomy 13 have extra fingers and toes (polydactyly). The feet may have prominent heels. In many cases, there are other health problems present at birth. These include heart defects, kidney problems, and/or an omphalocele (a condition in which some of the abdominal organs protrude through an opening in the abdominal muscles in the area of the umbilical cord). In males, the testes sometimes fail to descend into the scrotum. Females may have an abnormally shaped uterus, called a bicornuate uterus.

After birth, the physician usually takes a blood sample from a baby suspected of having trisomy 18 or 13 to perform a chromosomal analysis (called a karyotype). This confirms the physical findings of trisomy 18 or 13 and determines the underlying chromosomal abnormality. This information is important in determining the risk in future pregnancies. (Translocation and mosaic trisomy 18 and 13 have different recurrence risks). 

The physician may refer parents to a genetic physician or genetic counselor who can explain the results of chromosomal tests in detail, including what the recurrence risks may be in another pregnancy and what tests are available to diagnose chromosome problems before a baby is born.

Physicians and parents need to speak openly about the (in)appropriateness of intensive care procedures.

Many times parents are frightened and overwhelmed by all of the information associated with trisomy 18 and 13. Decisions surrounding the care of an infant with trisomy 18 and 13 are difficult and personal. There are many resources available to parents to help them during this time, including early intervention services, hospice care, social workers, the hospital chaplain or clergyman and genetic counselors. Families who have or have had a baby with trisomy 18 or trisomy 13 are particularly helpful and supportive since they have experienced many of the same questions and emotions.