Safest way to check cervix for preterm labor?

vermont7

I'm feeling crampy and my midwives want to check to see if I'm dilating and do the FFN test because I started dilating at 33 weeks with my first pregnancy (daughter was born at 38 weeks 1 day). Is it safer (won't cause dilation/cervical changes) to check cervix manually with fingers or with an ultrasound (vaginal or abdominal)?  

 

Also posted in April 2012 Moms