What size should I order? This is one of the most frequent questions I get, particularly with respect to presents for spring and summer baby showers. The warmer months are not typically thought of as "sweater season" so it's probably not a great idea to order a newborn sweater for a warm weather shower gift (at least not if you want the baby to actually wear the sweater!). But people often want the baby to be able to use their gift as soon as possible so deciding what size to get can be tricky.
A good approach is to think ahead to December, January, and February and determine how old the baby will be then. So, for a May shower, the baby is likely due in June or July, which means that he or she will be about six months in the winter. Ordering a six- to 12-month size will allow baby to wear the sweater as early as October or November (though it may be a bit big) and through those last cool days the following March and April.
Of course I'm always happy to help with any sizing questions.