Gauge: 18st in 4 in / 10cm in stockinette stitch
Yarn and needle size suggestions:
Hayfield Bonus Aran (100% Acrylic, 100g; 256m (280 yds)) and 5mm (8 US) knitting needles
Mary Maxim Best Value (100% Acrylic, 170g; 312m (341 yds)) and 5mm (8 US) knitting needles
Katia Merino Sport (100% Merino wool, 50g; 80m (87 yds)) and 5mm (8 US) knitting needles
Sirdar Supersoft Aran (100% Acrylic, 100g; 235m (257 yds)) and 5mm (8 US) knitting needles
Cascade Yarns Nifty Cotton (100% Cotton, 100g; 169m (185 yds)), and 4.5mm (7 US) knitting needles

You will need: yarn (up to 100 grams for the newborn size, up to 300 grams for the 18m size), knitting needles to obtain the gauge (a part of the romper is knit flat, then a small part of it is knit in the round, and then it is knit flat again), a tapestry needle or crochet hook to weave in the yarn ends, a stitch holder or a piece of scrap yarn and two stitch markers. A crochet book if you are making crochet ties.