Gauge: 17 st in 10cm/4in in stockinette stitch

Yarn and needle size suggestions:
DROPS DESIGN DROPS Alaska, (Wool (100%), 50g; 69m (75 yds)) and 5.5mm (9 US) knitting needles
DROPS DESIGN DROPS Big Merino, (Merino Superwash Wool (100%), 50g; 75m (82 yds)) and 5mm (8 US) knitting needles
LANG YARNS Merino Plus, (Merino Superwash Wool (100%), 50g; 90m (98 yds)) and 5.5mm (9 US) knitting needles
DROPS DESIGN DROPS Nepal, (Wool (65%), Alpaca (35%), 50g; 75m (82 yds)) and 5mm (8 US) knitting needles
LION BRAND Landscapes, (Acrylic (100%), 100g; 134m (147 yds)) and 5.5mm (9 US) knitting needles
CARON Big Cakes, (Acrylic (100%), 300g; 551m (603 yds)) and 5mm (8 US) knitting needles
RED HEART Soft, (Acrylic (100%)) and 5mm (8 US) knitting needles

You will need: yarn (up to 50 grams for the newborn size, up to 150 grams for the 12-18m size), knitting needles to obtain the gauge, darning needle to weave in the yarn ends and to graft the crotch.
The upper part of the romper is knit flat, then a small part of the romper is knit in the round, and then it is knit flat again.