No, they can’t. The uterus is not very large, and the baby takes up basically all of it.
In the first trimester, up to eight and rarely nine weeks, the baby may not be seen by an ultrasound. The abdominal scan may miss it, a transvaginal ultrasound will pick up the baby by the six weeks, but it also miss it in this period. But after this, that is, nine weeks there is no question of the scan missing the baby.
The scan sees right up to the posterior wall of the uterus. The baby resides in its sac, all the parts of the sac are visible to the scan. It CANNOT hide.