A uterine ultrasound generally does not make a woman niddah.
We recommend that you confirm with the physician that any instruments that enter the cervix are smaller than 19mm in diameter. If this is the case, then the procedure will not make you niddah.
During the days after the procedure, be careful about the laws of stains, as by wearing colored underwear, to head off unnecessary questions. Please see our articles on stains for more details and please get back to us with any further questions.
We wish you hatzlacha moving forward. You may find it helpful to arrange a free consultation with a Yoetzet Halacha Fertility Counselor, details here.
This response was updated on 25 August, 2021.