This pattern is written in American terminology.

BONUS:
Video tutorial on the example of size 9-12 months. If you have any questions or doubts, please watch the video. Good crocheting.

Overall pattern includes 4 sizes:

0-3, 3-6, 6-9 & 9-12 MONTHS.

Gauge:

24 stitches
and
26
rows sc (back loop only) =
10 x 10 cm (4 x 4 inch)

Materials:
Hook 3,5 mm & 3 mm. Yarn: 175m/50gr, 5 ply sport cotton/merino yarn (e.c. Drops «Safran», Drops “Baby Merino, Gazzal “Baby cotton”).
Color “A”: 200/220/235/250 gr.
Color “B”: 60/65/70/80 gr.

Romper is crochet in one piece. Seamless.