Can sitting in jacuzzi at 4 weeks pregnant for about 15 mins cause spina bifiba or brain defects?

Possibly: Neural tube defects occur in the 21-29 day of the pregnancy. Spina bifida tends to be as the result of issues taking place at the beginning of that window. Cleft palate, anencephally and cleft lip occur toward the end. The posterior neuropore closes about a week before the anterior neuropore. If the water was hot enough to raise your body temp it is possible. Get an ultrasound.