26 weeks pregnant and just had unprotected sex is it true that sperm can cause preterm labor?

No: If only that were so.... We could induce labor with sperm instead of medication! there have been extensive studies and there's no evidence that semen causes labor. If your doctor told you to avoid sex because of a problem with placenta, definitely avoid sex, but if not, it's okay. Make sure you're comfortable too, not feeling pressured, etc. Pregnancy can be confusing for couple's sex life.