Peritonitis is a set of symptoms of varying etiology, usually with an accompanying infection, systemic and local changes within the peritoneal cavity. Colonic diseases, especially colon perforation, are one of the most common causes of peritonitis. The course of the disease may be turbulent due to sudden perforation. In case of limited peritonitis the disease is not as acute as the perforation hole is small and it can be sealed by the omentum and internal organs. Abdominal pain is usually located around the source of infection and is less severe. A 38-year-old patient at 34 weeks gestation was hospitalized in the obstetric-gynecological ward of the Health Care Center with a diagnosis of preterm delivery, urinary infection and renal colic. Due to increasing peritoneal symptoms and deteriorating general condition of the patient, a decision to perform immediately exploratory laparotomy combined with the Cesarean section was made. The surgeon indicated a place in the left mesogastrium that could correspond with a drained interintestinal abscess and also found a large amount of fibrin in the lower floor of the peritoneal cavity. The initial point of the abscess remained unknown and the patient received total parenteral nutrition for 10 days. On 5 postoperative day the drain was removed from the peritoneal cavity and since day 10 patient health was steadily improving. Bacteriological cultures revealed abundant growth of E. coli that showed sensitivity to the used antibiotics. On 22 postoperative day the patient and her child were discharged home in good condition. Five months later, the patient was admitted to the surgical ward with acute abdominal pain with the diagnosis of an abscess in her left mesogastric and subgastric area, perforation of sigmoid diverticulum and fecal fistula. Our case illustrates great difficulties in diagnosing diseases of the abdominal cavity during pregnancy because causes and symptoms may be typical of this condition, as well as of unrelated diseases, including: kidney problems, appendicitis, cholecystitis and bowel disease. Examination of the pregnant patient presents a challenge and the symptoms are uncharacteristic. Tension of the abdominal wall, as well as the muscles of the digestive and urinary tract are reduced and the topography of the internal organs changes during pregnancy. The interpretation of laboratory tests becomes more difficult. In our case, the initial local peritonitis, caused by microperforation of the diverticulum, ran a latent course and was masked by both pregnancy and renal colic symptoms, consequently leading to diffuse peritonitis. The presented case demonstrates the importance of the problem and forces obstetricians to be more vigilant in determining the diagnosis and decision-making, because of its meaning for the health and even the life of the patient and her child.