I'm 37 years old. I had my first child 12 years ago; the pregnancy was fine, but labor was difficult. I am trying to have another baby now. I don't seem to have a problem getting pregnant, but I've had three pre-term labors before the 22nd week of pregnancy in the past four years. What could be causing this problem?
If your first labor was difficult, I wonder if there was any damage done to your cervix. If so, it's ability to hold in a pregnancy could be compromised; this is what we call an incompetent cervix. This complication can result in a preterm delivery mid pregnancy.
I recommend a vaginal ultrasound to measure the length of your cervix, and your doctor should compare this measurement with how your cervix feels on exam. If it is very short, then you're prone to having this happen again unless you have a cerclage.
If the cervix is fine, then I don't know why you've been having these problems, and your doctor may not be able to tell you either. But check the cervix, that's your best chance of discovering a cause.