Jerky and smooth movements during pregnancy are part of the normal development of a baby's neurologic and motor systems. There is some evidence, however, that frequent jerky movements can be signs of a neurologic problem. Consult your doctor if you notice a sudden increase in rapid fetal movements from your baby's usual pattern.
Development of Fetal Movement
The Handbook of Brain and Behavior in Human Development notes that movement emerges in the embryo between seven to eight weeks and includes jerky and smooth movements. Observations using ultrasound monitoring note that smoother movements become more common as the baby develops into the second and third trimesters.
According to a study and review published in 2011 in Frontiers in Psychology, the jerky movements you feel are more primitive and reflect early, immature neurologic, and other systems. Smooth, regular movements indicate maturation of the nervous, sensory, and motor systems and connections to the brain and spinal cord.
Origin of Jerky Movements
A baby's movements seen on ultrasound monitoring includes the whole body, head, eyes, mouth, chest, and limbs. According to the Frontiers in Psychology study of pregnant women at 26 and 36 weeks, the baby's jerky movements come mainly from:
- Generalized movements, such as a back-arch, a startle response, or a stretch
- Large and small limbs, including movement of hand to face
- Hiccups and vigorous breathing
In addition, it is possible that you might interpret normal fetal twitches or painless uterine cramps as jerky baby movements.
Second to Third Trimester Changes
During the second half of pregnancy, especially by 36 weeks, you will note fewer jerky movements as the baby is able to coordinate his muscles and make smoother movements. The Frontiers in Psychology study noted fewer jerky whole body movements, including startles and limb movements at 36 weeks compared to 26 weeks. Rapid fetal movement in the third trimester may also be a response to stimuli. The study also showed that at both 26 and 36 weeks, with intense stimulation from a loud noise, the baby will have more jerky, primitive movements of body and limbs than at baseline.
Movement and a Baby's Well-Being
According to the Royal College of Obstetricians and Gynecologists, a baby's movement during pregnancy is used as a standard measure of well-being. Babies who move less frequently may be abnormal, in distress, or at risk for stillbirth.
Jerky Movements and Well-Being
There is little information on what instances of jerky movements may be a sign of an abnormal or distressed baby during pregnancy. However, there are a few reports, such as the following, that found seizures on ultrasound monitoring of fetuses with an abnormal brain. These movements may be perceived as jerky.
- One article published in the Obstetrics and Gynecology, reported seizures as "rapid, repetitive" movements in a 32-week fetus who had brain cysts.
- Another report in the International Journal of Obstetrics and Gynaecology found sudden, jerky movements in a 32-week fetus with an abnormal brain.
- Finally, a study reported in Early Human Development found that anencephalic (without a brain) fetuses had clusters of "forceful, jerky movements" on ultrasound.
When to Consult Your Doctor About Rapid Fetal Movement
Jerky movements is one of the types observed in normal developing baby during pregnancy. However, if you notice more frequent, repetitive, or clusters of jerky movements than what you are used to, call your doctor or midwife to discuss your concerns and possible ultrasound monitoring.