Babies generally start to kick in the womb around seven weeks. Most women can feel the baby moving by the second trimester.
The first kicks felt by the mother is known as quickening. According to WebMD, a pregnant woman probably will not feel the baby move until week 16 and sometimes as far along as week 22. An ultrasound can catch these acrobatic movements on screen for the expecting parents to see.
The baby is continuously active, but during the second trimester, they are not yet strong enough for all of those movements to be felt by the mother. Women who have carried a baby before will be able to recognize the more subtle movements and be less likely to mistake them for hunger pangs or gas.