I took a cramp in my left foot 4 days ago,ever since my big toe has been numb. im currently 18 week's preg.

Localized swelling: With pregnancy, you want to watch for any swelling in the lower extremities. Combine that with hormonal changes that loosen the joints and create instability (perhaps in your case - the lower back and ankles). Swelling with instability in these joints can create some "pinching" of the nerves leading to the big toe. Usually self-limiting once delivery occurs. Talk to your OB about it too.