If the teat hole is round, milk may gush out when you turn the bottle upside down after preparing the formula. This happens because the warm milk causes the air in the bottle to expand, pushing the milk out. After a while, it will settle, and the milk will drip instead – this is when you can feed your baby. To reduce the chance of gushing, try unscrewing the cap briefly after preparing the formula and then screw it back on to lower the air pressure in the bottle.