The same abdominal pain differential as non-pregnant patients, plus:
Ectopic pregnancy
First trimester abortion
Complete abortion
Threatened abortion
Inevitable abortion
Incomplete abortion
Missed abortion
Septic abortion
Round ligament stretching
Incarcerated uterus
Malposition of the uterus
Complete abortion
Threatened abortion
Inevitable abortion
Incomplete abortion
Missed abortion
Septic abortion
Labor/Preterm labor
Placental abruption
Placenta previa
Vasa previa
Uterine rupture
Vaginal trauma
HELLP syndrome
Cholestasis of pregnancy<unk> 
Incarcerated uterus
Acute fatty liver of pregnancy
Malposition of the uterus
Placenta accreta
Placenta increta
Placenta percreta
Hemorrhagic ovarian cyst
Fibroid degeneration or torsion
Ovarian torsion
Constipation
Extra-amniotic infections such as pyelonephritis, appendicitis, pneumonia
Chorioamnionitis
The same abdominal pain differential as non-pregnant patients, plus:
Septic abortion
Ectopic pregnancy
Postpartum endometritis
First trimester abortion
Preterm labor
Complete abortion