What are possible pregnancy complications?

Many. Pregnancy complications include bleeding, miscarriage, premature labor, preterm delivery, pre-eclampsia, gestational diabetes, fetal anomalies, etc, etc - the list is far too long for a complete coverage here. If you have a specific concern list it for a definitive answer.
Multiple. Miscarriage, stillbirth, fetal anomalies, multifetal pregnancies, preterm birth, preeclampsia/eclampsia, hypertension, diabetes mellitus, cholestasis, pemphigoid, pyelonephritis, malpresentation and cesarean section, atony, hemorrhage, previa, accreta, maternal/neonatal death are some of the possible pregnancy complications.