The Question
What condition could lead to a baby being born without a full limb? Is it a question of genetics in the parents (one or both) or a condition in the womb? My character has a son whose arm ends a few inches past the elbow. He is seven at the start of the novel and can manipulate many objects deftly although obviously cannot do activities that require two hands. I want to explain to the reader since this condition affects much in his mother’s life.
The Answer
Great question, thanks for sending it!
Amelia and phocomelia are congenital conditions, meaning they are present at birth. In Latin, “a” means ‘without’, and “melos” means ‘limb’, so the name literally means “without limb.” Phocomelia is the term used for limbs that are shortened. The Latin meaning of the prefix “phoco” is ‘seal,’ so phocomelia technically means ‘seal limbs’ but I don’t think anyone would appreciate it being called that.
Amelia or phocomelia can affect any number of limbs and can range in severity from the loss of a single finger to the total absence of a limb. Tetra-amelia syndrome, the total loss of all four limbs, is quite rare. Amelia and phocomelia can be caused by several different factors.
The first is exposure to thalidomide. Back in the 1950s and 1960s, thalidomide was a drug used to treat nausea in pregnancy. Turns out, it was a teratogen that resulted in a host of birth defects, including severe phocomelia. It’s since been blacklisted as a drug to use during pregnancy, though it is still used to treat leprosy and some cancers.
Another cause is a genetic syndrome. There is a cluster of rare genetic disorders associated with amelia and phocomelia. One, called Roberts Syndrome, is an autosomal recessive disorder, meaning that both parents must pass down an altered copy of the gene for the child to show symptoms. This means that at least one grandparent on each side had the gene as well. Tetra-amelia syndrome is another rare autosomal recessive condition that causes the total loss of all four limbs due to a defect in the WNT3 gene. Another genetic syndrome, called Cornelia de Lange Syndrome (CDLS for short), can be caused by a constellation of different genetic anomalies. Some of the genes for CDLS are inherited, like Roberts Syndrome, but some are sporadic, meaning that the gene mutation occurred during embryonic development.
This leads me to the last, and murkiest, cause of phocomelia: random factors affecting early pregnancy. This can include drug and alcohol use (especially during early pregnancy), gestational diabetes, radiation (usually requiring a higher level than your regular chest X-ray), viral infections, decreased blood flow to the placenta, or even tight amniotic bands wrapped around the fetus’s limb, preventing its growth. Most of the time, doctors aren’t able to pinpoint a single reason.
Since your character has unilateral phocomelia (a single arm ending just below the elbow) and no other genetic anomalies (and wasn’t born in the ’50s or ’60s), he most likely falls somewhere in that third category. Not a very satisfying answer, but – like so many things in medicine (and especially pregnancy!) – that’s all we know for sure.
If you want to read more, here are a couple of great articles from Healthline, the National Organization for Rare Disorders, and the Dana Ferber Cancer Institute.