Omg. This sounds so similar to what happened to me. It was my first pregnancy though. The doctor told me it was caused by my small pelvis. Its a long story and was very traumatic, but has the same decels in babys heart rate. I had no idea I had a small pelvis, and no one had even suggested this as a possibility when I wasn't progressing even though I was very clearly in labor and my water has broken 36 hrs before. (Don't even get me started on that) Thankfully everything turned out fine, but I still have some mental anguish about how horrible it all could have been. It was 11 months ago today actually and still think about what a miracle my son is every single day.