Twins in our study were born
at an average of 36 weeks, but they ranged from 25 weeks
to 40 weeks. Thirty-eight weeks is considered full-term
for a twin pregnancy; forty weeks is considered full-term
for a singleton pregnancy. We consider twins premature
if they are born before 36.5 weeks.
On average, twins in our study first began smiling at
about 8.5 weeks. However, twins' first smiles range
(quite broadly) between 2 weeks and 15 weeks. Full term
twins, on average, first smile socially around 7.5 weeks,
while premature twins began to smile at an average of
9 weeks. Although premature and full-term twins smile
at significantly different times than singletons, twins
are still within the same range of 6-10 weeks.
The twins in our study first crawled on hands and knees
at an average of 38 weeks (or 9 months). Premature twins
first crawl on hands and knees, on average, at just over
40 weeks of age (or 9.5 months old), whereas full-term
twins crawled, on average, just before 36 weeks of age
(or 8.5 months old). Once again, twins are similar to
singletons, who begin crawling at an average of 7 months
(with 90% of singletons crawling between 5 and 11 months).
Just as with smiling and crawling, twins have been found
to walk at a similar age as singletons. On average, twins
walk at 12.75 months and singletons at 11.75 months. Premature
twins do not significantly differ from full-term twins.
Twins in our study said their first word at 54 weeks
(or 12.75 months), with no significant differences for
premature twins. Singletons around the world also say
their first word around one year of age, although most
children babble beginning around 6 months old.