Intrauterine Growth Restriction

This is my first pregnancy and I am 20 weeks today.  My tri-screen tested positive for spina bifida protein and I was scheduled for an ultrasound.   At the U/S the doctor stated that there are no issues with the dates and spina bifida was ruled out.  He stated the baby is structurally fine except that she is measuring 2.5 weeks behind and that there is low of levels of amniotic fluids.  He said he has never seen (Intra-uterine Growth Restriction) this early before @18 weeks.  He claims it may be a bad placenta.   I was confused and asked a lot of questions however, my fears were not alleviated.  I am 26 years and I have a slight heart murmur as well as several uterine fibroids.  He stated that he does not think any of these conditions have anything to do with the baby’s slow growth rate.  Furthermore, as I read online to educate myself on this condition, I notice none of the factors really applied to me, such as obesity, drug use, or diabetes.  My blood pressure was measuring high but not high enough to be a concern.   I was ordered to keep track of it at home and it has consistently been normal.  The ultrasound tech said the baby was about 4cm and I think she was supposed to be 7cm at that time.  The doctor said he does not think its Down Syndrome or Trisomy 18 and said an amniocentesis can be taken to rule this out.  He also said this is very bad and the baby will not likely survive and he mentioned possible termination.  Usually people with this condition are induced to remove the fetus from the bad environment, however, because I am so early this is not an option for me.   I have never felt her kick, my boyfriend tries to reassure me that first time mothers can take about 25 weeks to feel kicks but I don’t think I should have to wait that long.  I eat regularly and have never missed a day of my prenatal vitamins.   If given the option I will do the amnio to rule out any genetic factors, however, I don’t want to have to make the horrible decision of having to terminate this pregnancy.  If anyone has similar experiences please share.  I have my second U/S tomorrow to determine how she is growing.  Thank you.