According to Fetal Movement
According to an overview produced by the Royal College of Physicians of Edinburgh, purposive movement begins at about 18 weeks, gradually replacing reflex movements
but in the same article says
At week 11, the fetus can open its mouth and suck its fingers; at week 12, it begins to swallow amniotic fluid.
I highly doubt that open a mouth and suck its fingers can be classified as "reflex movements", then how it is asserted that there is not purpose movement in the fetus until week 18 when a fetus can open its mouth and suck its fingers at week 11?