The most likely diagnosis is preeclampsia, which is classically defined as new-onset hypertension after 20 weeks of pregnancy with proteinuria (≥300 mg/24 h or a urine protein-creatinine ratio ≥300 mg/g). According to new guidelines, preeclampsia can also be diagnosed in patients without proteinuria if the hypertension is accompanied by one of the following conditions: thrombocytopenia (platelet count <100,000/µL [100 × 10⁹/L]), kidney dysfunction (serum creatinine concentration >1.1 mg/dL [97.2 µmol/L] or a doubling of the serum creatinine concentration in the absence of other kidney disease), impaired liver function (elevated blood concentrations of liver aminotransferases to twice the normal concentration), pulmonary edema, or cerebral or visual symptoms. Because this patient meets the criteria for blood pressure elevation and also has thrombocytopenia and possibly mild pulmonary edema, the diagnosis of preeclampsia may be made without the presence of proteinuria. She has several risk factors for preeclampsia, including advanced maternal age and first pregnancy. Other risk factors for preeclampsia include family history, diabetes mellitus, obesity, chronic kidney disease, and twin gestation.

Chronic hypertension is defined as systolic pressure ≥140 mm Hg and/or diastolic pressure ≥90 mm Hg that existed before pregnancy, is present before the 20th week of gestation, or persists longer than 12 weeks postpartum. This patient had normal blood pressures prior to her most recent visit, making chronic hypertension unlikely.

The diagnosis of HELLP (hemolysis, elevated liver enzymes, and low platelets) syndrome requires evidence of hemolysis and abnormal liver chemistry tests in addition to thrombocytopenia.

Normal hemodynamic changes occurring during pregnancy include an increase in cardiac output and reduction in both systemic vascular resistance and systemic blood pressure. Therefore, the significant blood pressure elevation seen in this patient who was previously normotensive is not considered to be a normal change associated with pregnancy. Crackles, an S₃ gallop, and an elevated central venous pressure would not be expected findings during a normal pregnancy and reinforce the diagnosis of preeclampsia.