Association Between Self-reported Prenatal Cannabis Use and Maternal, Perinatal, and Neonatal Outcomes – PubMed

Among pregnant women in Ontario, Canada, reported cannabis use was significantly associated with an increased risk of preterm birth. Findings may be limited by residual confounding.

Source: Association Between Self-reported Prenatal Cannabis Use and Maternal, Perinatal, and Neonatal Outcomes – PubMed

TL;DR: don’t use cannabis while pregnant.  Probably a good idea if you’re “trying”.