Preeclampsia is a serious hypertensive disorder that develops during pregnancy, typically after the 20th week of gestation. This condition involves high blood pressure and can affect various organ systems, including the kidneys and liver. While standard ultrasound cannot diagnose preeclampsia in the mother, it is an indispensable tool for assessing the effects of the disease on the developing baby and the placenta. The imaging provides information about how the condition is impacting the pregnancy, guiding management for both the mother and the fetus.
The Primary Diagnostic Tools
The definitive diagnosis of preeclampsia relies on non-imaging methods that assess the mother’s physiological status. Diagnosis occurs when a pregnant person develops new-onset hypertension (systolic blood pressure of 140 mmHg or higher or diastolic pressure of 90 mmHg or higher, measured on two occasions at least four hours apart after 20 weeks). This high blood pressure must be accompanied by signs of organ dysfunction or proteinuria.
Laboratory tests are the definitive diagnostic standard, not imaging. These tests include urinalysis to measure protein in the urine, such as a protein-to-creatinine ratio of 0.3 or greater. Blood work also checks for signs of liver impairment (elevated liver enzymes) and assesses platelet count. These measurements establish the diagnosis and severity of the maternal disease, while ultrasound focuses on the fetal consequences.
Monitoring Fetal Well-being
Once preeclampsia is suspected or confirmed, standard ultrasound becomes the primary method for monitoring the baby’s health. The condition often causes placental dysfunction, which can restrict the flow of nutrients and oxygen to the fetus. Frequent biometry measurements, such as the fetal head, abdomen, and femur length, are taken to estimate fetal weight and track the baby’s growth trajectory.
These measurements help identify Fetal Growth Restriction (FGR) or if the baby is Small for Gestational Age (SGA). FGR is a common complication of preeclampsia and signals the need for closer surveillance or intervention. The ultrasound assesses the volume of amniotic fluid surrounding the baby, quantified using the Amniotic Fluid Index (AFI). Low amniotic fluid, a condition called oligohydramnios, is a sign of placental insufficiency, as reduced blood flow to the baby can lead to decreased urine output.
Doppler Studies and Blood Flow Assessment
A specialized type of ultrasound called Doppler velocimetry is the most powerful imaging tool for assessing the physiological impact of preeclampsia. Doppler technology uses sound waves to measure the speed and direction of blood flow through specific vessels. This technique evaluates the resistance to blood flow in both the maternal-to-placental circulation and the placental-to-fetal circulation.
The Uterine Artery Doppler examines the maternal blood flow supplying the placenta. An abnormal result, characterized by an elevated Pulsatility Index (PI) or Resistance Index (RI), indicates high resistance in these vessels. This high resistance suggests that the normal transformation of the uterine blood vessels has been incomplete, signaling impaired blood supply.
The Umbilical Artery Doppler assesses the blood flow from the placenta to the baby. Normally, this artery shows continuous forward flow during the entire cardiac cycle. An increase in the Systolic/Diastolic (S/D) ratio, or the absence or reversal of end-diastolic flow in severe cases, indicates significant placental compromise and fetal distress. Absent or reversed end-diastolic flow is a serious finding that may prompt immediate delivery, regardless of gestational age.
Ultrasound in Early Risk Prediction
Beyond monitoring an established case, ultrasound is integrated into screening protocols for predicting preeclampsia earlier in pregnancy. This screening is typically performed in the first trimester, between 11 and 14 weeks of gestation, for women identified as high-risk.
The screening combines an early Uterine Artery Doppler assessment, which measures blood flow resistance, with maternal blood markers. The key blood markers include Pregnancy-Associated Plasma Protein-A (PAPP-A) and Placental Growth Factor (PlGF). When the Uterine Artery Pulsatility Index is elevated and the blood markers are abnormal, the combination can accurately identify women who will later develop pre-term preeclampsia. This early identification allows for prophylactic interventions, such as starting low-dose aspirin before 16 weeks, which reduces the incidence and severity of the disease.