The rules are easy: guess which baby is which in each group. Some groups have only two babies. They are not necessarily exactly the same age in these photos, just roughly the same size so they could fit in the same outfits. Click to enlarge.
If you can't tell, don't feel bad -- I'm their mother and I don't see any differences either.
Group 1 - stripey shirt
Group 3 - smashed newborn faces
Group 4 - blue jammies