The biophysical profile is often combined with fetal heart rate testing, which can take an additional 20 to 40 minutes. Doctors will request a biophysical profile if the baby isn't growing as he should, if you have diabetes or high blood pressure, or if you're still pregnant after your due date, to name a few common reasons. Babies who are deprived of oxygen don't move normally, so the test is a way to see if he's getting enough oxygen. Dehydration may reduce the amount of amniotic fluid that you produce, another good reason to remember to drink six to eight glasses of water daily. If you haven't eaten in a while and your blood sugar is low, that may reduce your baby's activity and particularly his breathing during a biophysical profile exam.
Babies follow circadian rhythms, and as any pregnant woman knows, are more active at night.
Babies make a lot of sucking movements in utero, but I've never seen one actually suck her thumb. But that's not surprising as it takes a while after birth for babies to learn that behavior. The most important thing for you to know is your baby's pattern of behavior. Some are active in the morning, others at night. As you get into the third trimester it is a good idea to pay attention to your baby's movements every day.