How can there not be purpose movement in the fetus until week 18, when a fetus can open its mouth and suck its fingers at week 11?
Swallowing (Nishino, 2013) and sucking (rooting reflex) are reflexes. The fingers may just happen to be in the vicinity of its oral region (they often are; Fig. 1.), and oh boy are babies keen to grab a nipple when it's close! Mouth openings can henceforth be also reflexive, being part of the rooting reflex.


Fig. 1. Fetal hands in facial area. sourceL: ABC News

Reference
- Nishino, Front Physiol (2012); 3: 4892013