One baby is crawling, and the other doesn't even seem interested. One baby has four teeth, the other has none. One baby is talking in sentences, and the other says only a few words. Comparing twins' development is inevitable. But when should you be concerned about the baby who develops "behind schedule"?

Most parents should not be concerned. The vast majority of twins will develop normally, on their own schedule, in their own way. If one baby is a few weeks behind the other, there is usually no reason for worry.

Many twins, due the fact that they are born prematurely and/or low-birthweight, meet developmental milestones later than average. Doctors often refer to twins' "adjusted age", which means figuring their age from their due date versus their delivery date. If your babies are meeting milestones within a month of their adjusted age, most pediatricians will tell you not to worry. For peace of mind, parents should be aware of less common developmental delays related to prematurity. See the links at the end of the article to read more on this topic.

The most documented developmental delay amongst twins is language development (talking). While there are many theories as to why twins are late talkers and/or disinterested in learning how to talk, chances are that the reasons are many and varied. If one or both of your twins seems to be lagging behind in speech development (not speaking at all by 18 months, for example), be sure to discuss this with your pediatrician, who will probably refer you to a specialist.

Two more serious developmental delays are also believed to be more common amongst twins than the general population.

Autism
Several studies have found that autistic spectrum disorders are more common in both identical and fraternal twins than the general population (though identical twins are more at risk). Even in identical twin pairs, however, one twin may have an autistic disorder and the other may not. For information on the warning signs of autism, see the link at the end of the article.

Cerebral Palsy
This disorder has been studied extensively, and twins tend to suffer from it more often than the general population. Other factors associated with twins, such as advanced maternal age, low birthweight, and prematurity, also put a child at risk for cerebral palsy. For information on the warning signs of cerebral palsy, see the link below.

Lasting Effects of Prematurity: https://www.bellaonline.com/articles/art42713.asp
Autism: Learn the Signs: https://www.autismspeaks.org/
Cerebral Palsy Risk Factors: https://www.about-cerebral-palsy.org/diagnosis/risk-factors.html
BabyCenter: Warning Signs of a Physical Delay: https://www.babycenter.com/general/baby/babydevelopment/6720.html