The ratio decreases towards term, from about 3.5 at 24 weeks to about 2.2 at term.
As the diastolic flow becomes diminished in fetal compromise, the systolic/diastolic ratio will rise.
The UA ratio at 24 weeks in a fetus with severe intrauterine growth retardation. A represented the systolic value and B the end-diastolic value. The ratio was 4.6.
The UA ratio at 28 weeks. The ratio had increased to 5.4.
The UA ratio at 32 weeks. The ratio had increased to infinity because of an absence of end-diastolic flow. The fetus is at tremendous risk of intrauterine death.