What causes 22week fetus' HR to drop from 150 to under 40bpm??

My obstetrician sent me to a maternal fetal medicine specialist because at 22 1/2 weeks my baby's heart rate is dropping from about 150 to less then 40 bpm about every 3-5 minutes, it lasts about 20 seconds and then starts to climb back to normal.  The specialist determined that the episodes are both induced by pressure and spontanious. The babies heart structure is perfect and is growing normally and my placenta is in front of the baby, or closest the outside wall. There were no signs of umbilical cord compression. The episodes happen when the baby makes a big movement or the Doctor pushes on my belly. The Doctor's first thought is that the baby has vagus nerve over-sensitivity but says she has not had a fetus have this frequent of episodes nor any spontanious episodes.  She is testing me for Sjogrens, though it is unlikey, and says that heart block doesn't have induced decelatations.  It sounds to me like what we are seeing doesn't fit either of her theories.  Both the specialist and my obstetrician say that at this point in gestation there is nothing they can do. I can't find any literature or personal stories that resemble mine. Can you provide any additional thoughts to this??  Could the problem be with the placenta perhaps?

[Bridget A. Hewitt, RN]

Hi, I would try posting this in the pregnancy expert forum as well.  

[JoyRenee]

A heart rate below 100 is definitely cause for concern. Though I have no idea why this could be happening unless there is a heart defect undetectable by ultrasound. I wish I had more answers but I don't.

[martikadragoon]

Most likely, if baby makes it to delivery, he/she will need a pacemaker to be sure that the heart beats normally.

They may choose to do fetal monitoring weekly or daily when you hit 30+ weeks... to be sure baby's heartbeat isn't slowing too much.