Miscarriage: This is one of the most common causes of abdominal pain in the first trimester, often accompanied by vaginal bleeding. The pain can range from mild to severe and is usually crampy in nature.

Ectopic Pregnancy: Although less common than miscarriage, ectopic pregnancy is a significant cause of abdominal pain in early pregnancy. It is characterized by sharp, stabbing pain, often unilateral, and can be accompanied by vaginal bleeding.
Ovarian Cyst Rupture or Torsion: These conditions can cause sudden, severe abdominal pain and are more likely in pregnancy due to the increased size of the ovaries and the potential for cyst formation.
Urinary Tract Infection (UTI): UTIs are common in pregnancy and can cause abdominal pain, usually accompanied by dysuria, frequency, and urgency.
Constipation: Pregnancy hormones can cause constipation, leading to abdominal discomfort or pain.

Appendicitis: Although less common in the first trimester, appendicitis can occur and is a surgical emergency. The pain typically starts near the navel and then moves to the lower right abdomen.
Placental Abruption: This condition, where the placenta separates from the uterus, can cause severe abdominal pain and vaginal bleeding, and is a significant threat to both mother and fetus.
Ureteral Colic (Kidney Stone): While not as common, kidney stones can cause severe, colicky pain and can be a significant issue in pregnancy due to the potential for urinary tract obstruction.

Corpus Luteum Cyst Rupture: A rare cause of abdominal pain in early pregnancy, usually associated with a history of assisted reproduction.
Adnexal Torsion: Torsion of the ovary or fallopian tube, which can cause severe abdominal pain and is more likely in pregnancy due to the increased size of these structures.
Gallbladder Disease: While gallstones are common in pregnancy, they rarely cause symptoms in the first trimester. However, they can lead to severe abdominal pain if they obstruct the cystic duct.