It depends, but you could use the method my doctors office recommends. Every day at what you feel is the most active time for the baby (maybe after a lunch or dinner?) you sit and wait for the baby to move. Record how long it takes for the baby to move 10 times. It could take 2 minutes, and it could take 20 minutes. But if it takes longer than an hour to reach 10 movements in the time youre actively paying attention, call the doctor.
Dont know if that helps, but im 28 weeks and they 'require' their patients to do this and fill out a chart to go along with it. I think thats a bit much, but whatever.
at 5:07 PM on Nov. 14, 2009