Why hasn't the fontanel closed on our 22-month-old twins?
The average age a fontanel closes is 12 to 18 months. Some close as early as 9 months and some not until age 2. The most important measurement is head size: Are your boys' heads growing at a normal rate? Is the fontanel flat or slightly concave? If the answer to both questions is yes, there is probably nothing wrong. The worrisome thing is whether there is some abnormal pressure inside the head that is causing the bones to stay apart, but it sounds as if this is unlikely in your healthy, normal boys.
Prematurity and being abnormally small for gestational age at birth
may cause delayed closure. It is likely that your twins were at least a little premature. Thyroid problems and some abnormalities of bone development may contribute as well. If the fontanel is not showing signs of closing at age 2, your pediatrician may take some X-rays and blood tests just to be sure nothing is wrong.