Cervical Length Monitoring During Pregnancy: A Personal Experience

Question
I am 33 years old.  I have a 5 year old son.  I had a m/c at 6 weeks in June 05.  I am currently 20 weeks pregnant with a due date of 12th June 06.  When pregnant with my son, my cervix was being monitored due to possible exposure to DES while inutero.  At 14 weeks, an ultrasound showed that my cervix was 0.8 cm and a stitch was inserted.  This was removed at 37 weeks and I went into labour at 40 weeks and 3 days.
My current ob does not feel that I actually had an incompetent cervix due to the fact that I went to term (and also had a long labour).  Also, the length of my cervix is being monitored fortnightly with transvaginal ultrasounds.  So far the figures have been good, ranging from 4.5 to 4.9 cm.  Today it was 4 cm. My ob is still not worried.  My questions to you are:
1) do you agree that a vaginal ultrasound may show an incorrect result (thinking here about the ultrasound done with my son)
2) should I be worried that last week the length was 4.8 cm and this week it is 4 cm.  My ob feels that this is just due to normal variation during the procedure.
3) when does the length of the cervix start to shorten or change during a normal pregnancy.  ie.  is some of this variation due to a normal shortening for my stage of pregnancy?
4) do you agree that is is unlikely that my cervix was incompetent when I had my son because I went to term?

Thanks.  Sorry for all these questions.  Its just nice to have a second opinion.

Answer
I doubt that you had an incompetent cervix because you did reach term. However you had a stitch at 14 weeks. When you have a stitch and it is removed, sometimes you will not immediately to into labor. It is doubtful that your cervix was only 0.8cm in length (because it is now 4cm). Vaginal ultrasounds are good, but vary based on the experience of the sonographer. Perhaps the 0.8cm was an error. Do not worry until the cervical length is less than 3cm, which may indicate the beginning of preterm labor. I agree with your obstetrician. I would have serial sonography done to continue assessing your cervical length, but I doubt that there will be a problem. Good luck.