The ratios (indices) of the resistance to blood flow (impedance) are calculated
using the following formulas:
S/D ratio = (systolic / diastolic ratio)
Resistance index (RI) = (systolic velocity - diastolic velocity / systolic velocity)
Pulsatility index (PI) = (systolic velocity - diastolic velocity / mean velocity)
In the normal fetus, the resistance to flow
(impedance) decreases in the umbilical artery due to increased numbers of tertiary stem villi
as the placenta matures.
Normal Reference Ranges
The umbilical artery (UA) impedance indices increase when there is decreased
end-diastolic flow due to reduced placental perfusion and "utero-placental
insufficiency" as is seen in intrauterine growth restriction (IUGR). Absent or reversed UA
end-diastolic flow are particularly ominous findings.
The flexible tube that connects the fetus at the
abdomen with the placenta. The umbilical cord contains two umbilical arteries
which carry blood depleted of oxygen from the fetus to the placenta, and one
umbilical vein that carries blood enriched with oxygen from the placenta to the
The blood vessel that carries oxygen enriched blood from the placenta to the fetus.
Both members of a chromosome pair are
inherited from one parent rather than one from each parent. Uniparental disomy
may be maternal or paternal.
Recurrent tightening and relaxation of the uterine wall
A cut made into the wall of the uterus.
A tear through the entire thickness of the uterine wall.
Opening of a surgical scar on the uterus where the visceral peritoneum stays intact is referred to incomplete rupture or dehiscence.
The pear-shaped reproductive organ in a woman's pelvis. The lower narrow part of the uterus (the cervix) opens into the vagina. The
uppermost part of the uterus is connected on both sides to the uterine
The uterine tubes, uterus, and upper vagina are formed by fusion of the two paramesonephric (Müllerian) ducts beginning at about 10 weeks gestation. Fusion of the ducts with resorption
of the uterine septum is usually completed by about 20 weeks' .