Onesies are little outfits like tshirts that snap in the crotch area. This is wonderful so that the shirts and sweaters don't pull up leaving that precious skin open to the elements.
When I was sewing machine shopping I found this
http://www.oksewingandvacuum.com/elnapro.htm It is a free pattern for how to make onesies, even though it says Elna you can make it on any machine, just scroll down a bit till you see it.
When I was pregnant with dd I searched and searched for these. I didn't like the styles of the month and finally found nicely made ones at an outlet store. They had little embroideries just below the center of the neckline little baby animals and said things like "daddy and me" "I love my mommy" The thread on the sleeves matched the embroidery. I just loved them. They were perfect for wearing out, wearing in, or layering with.