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.