After the fourth week of embryonic development (which is the sixth week from the last period), when
the spinal column closes, there is no particular need for extra vitamins
. It is reasonable at that point to stop taking vitamins until the nausea has passed. If you really want to take some sort of vitamin, a children's chewable kind sometimes goes down easier.
The downside to vitamin supplementation
It's unhealthy to take more than one multivitamin a day.