Expansion of the plasma volume and an increase in red blood cell mass begin as early as the 4th week of pregnancy, peak at 28 to 34 weeks of gestation, and then plateau until parturition (4,5). Plasma volume expansion is accompanied by a lesser increase in red cell volume (6). As a result, there is a modest reduction in hematocrit, with peak hemodilution occurring at 24 to 26 weeks. The blood volume in pregnant women at term is about 100 mL/kg (7). The cardiac output rises 30% to 50% (1.8 L per minute) above baseline during normal pregnancy in part from changes in three important factors that determine cardiac output: preload is increased due to the associated rise in blood volume, afterload is reduced due to the decline in systemic vascular resistance, and maternal heart rate rises by 15 to 20 beats per minute (8).

During pregnancy, white cell volume increases from 5,000 to 6,000 cells/mm³ to 16,000 cells/mm³ in the second and third trimesters and to 20,000 to 30,000 cells/mm³ in early labor (9). The glomerular filtration rate (GFR) and renal blood flow rise markedly during pregnancy. The increase in GFR can be demonstrated within 1 month of conception and reaches a peak approximately 40% to 50% above baseline levels by the end of the first trimester, resulting in a decrease in blood urea nitrogen and serum creatinine as well as glucosuria despite normal plasma glucose levels (10). Both kidneys increase in size by 1 to 1.5 cm during pregnancy primarily due to an increase in renal vascular and interstitial volumes (10). Ureteral dilatation during pregnancy results from hormonal effects, external compression, and intrinsic changes in the ureteral wall (11). Physiologic hydronephrosis typically occurs and is more common on the right than the left side (90% versus 10%).

In the respiratory system, the diaphragm rises by up to 4 cm, and the chest diameter can increase by 2 cm or more (12). Diaphragmatic excursion is not limited by the uterus and actually increases by up to 2 cm. Pregnancy is a state of relative hyperventilation, which may be centrally mediated through progesterone. The respiratory rate does not change while the tidal volume increases, resulting in an approximately 50% increase in minute ventilation. The normal PaO₂ in pregnant women ranges from 100 to 110 mm Hg (13,14,15,16). In addition, there is a decrease in arterial carbon dioxide levels (PCO₂ and PaCO₂) from the nonpregnant average of 40 mm Hg to a plateau of 27 to 32 mm Hg during pregnancy (14,15). This respiratory alkalosis is followed by compensatory renal excretion of bicarbonate, so that the resultant arterial pH is normal to slightly alkalotic (usually 7.40 to 7.45) (16). The decrease in PaCO₂ probably helps the fetus to eliminate carbon dioxide across the placenta.

Pain perceived from the abdomen is caused by peritoneal irritation, mechanical stretching, or ischemia (17). Physical exam findings in pregnancy may be blunted compared with those of nonpregnant patients with the same disease or process. Peritoneal signs may be absent due to the lifting and stretching of the anterior abdominal wall in pregnancy, preventing contact with the parietal peritoneum and eliminating rebound tenderness or guarding (18). In addition, the gravid uterus may distort the clinical picture by obstructing the movement of the omentum to an area of inflammation. Performing an examination with a pregnant patient in the left or right decubitus position, displacing the uterus to one side, may be of some benefit in certain clinical situations.

Diagnostic imaging is often necessary during pregnancy. Sonographic examination is a common occurrence in pregnant women, but other types of radiological evaluation may also be required. Although the safety of radiation exposure during pregnancy is a common concern, a missed or delayed diagnosis can pose a greater risk to the woman and her pregnancy than any hazard associated with ionizing radiation (19). In many cases, the perception of fetal risk is higher than the actual risk (20,21). Table 2.1 provides an overview of the total fetal exposure of ionizing radiation in some commonly performed radiologic studies. Ionizing radiation can result in the following three harmful effects: (a) cell death and teratogenic effects, (b) carcinogenesis, and (c) genetic effects or mutations in germ cells (22). The threshold at which an increased risk of congenital malformations is observed in radiation-exposed embryos/fetuses has not been definitively determined. The best evidence suggests that the risk of malformations is increased at doses above 0.10 Gy, whereas the risk between 0.05 and 0.10 Gy is less clear (23). The American College of Obstetricians and Gynecologists makes the following recommendations for the use of diagnostic imaging techniques in pregnancy (24):

The differential diagnosis of acute abdominal pain in pregnancy includes etiologies similar to those in the nonpregnant patient as well as causes specific to the pregnancy (Table 2.2). The following sections review common causes of acute abdominal pain in pregnancy and detail safe diagnostic strategies for both the mother and the fetus.