Is it safe to drink miralax (polyethylene glycol) when pregnant? Will it cure constipation?

Constipation. Change in bowel habits, either diarrhea or constipation can occur with pregnancy. Most critical to bowel function is eating regularly and following a careful diet. When constipation develops, increasing water intake and adding fiber (metamucil, citrucel, benefiber) is usually enough. If okay with your obgyn, MiraLAX (polyethylene glycol) is usually well tolerated and can help if fiber and water are insufficient.
Category C. Miralax (polyethylene glycol) is category c for pregnancy. Risk can not be ruled out- adequate, well-controlled human studies are lacking, and animal studies have shown a risk to the fetus or are lacking as well. There is a chance of fetal harm if the drug. Besides, laxatives may produce uterine contractions. Discuss it with your obstetrician.
Yes. Miralax (polyethylene glycol) is an osmotic laxative it attracts water into the stool which makes it swell giving you the urge to pass it. It will 'relieve' constipation if that is what you are asking. An actual 'cure' requires a determination of the cause. It is safe for you to use in pregnancy so long as you do not use it frequently. If you overuse it you may cause a dangerous imbalance in your electrolytes, .