Most baby sweaters are cardigans, split up the front, because it's really hard to put a pullover on a wiggly baby. But you can still knit it seamlessly. A top down raglan or yoke is easiest, you can search for the 5 hour baby sweater, also called the Quick baby sweater. A pattern generator can be found at
www.knittingfool.com where you put in your gauge and the chest size you want and it makes a pattern for you. There's also a freebie at
Knittingpure and Simple.