A woman in labour, on vaginal inspection: cervix dilation is up to 2 cm, fetal bladder is intact. Sacral cavity is free, sacral promontory is reachable only with a bent finger, the inner surface of the sacrococcygeal joint is accessible for inspection. The head of the fetus presents. Sagittal suture occupies the transverse diameter of pelvic inlet, the small fontanel to the left, on the side. What labour stage is this?