A seven and a half month preemie will usually sit still around 8 months and roll over at 7 months. Normal full-term babies can sit on their own with their hands for a while at about 6 months of age, but they have to wait until 8 months of age to sit still. Normal full-term babies can roll over consciously and switch between the supine and prone positions at 7 months of age. Preterm babies, whose growth and development may be delayed compared to full-term babies, may be able to sit still at around 8 months and roll over at 7 months. Or the time may be a little longer, pregnant mothers can not worry too much. If the baby cannot sit up or turn over for a long time, it is recommended to go to the hospital for relevant examinations to clarify whether there are congenital diseases, etc.