Pregnancy Terms Glossary
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Umbilical arteries

Blood vessels originating from the fetal internal iliac arteries that carry all the oxygen depleted  blood from the fetus through the umbilical cord to the placenta.

Umbilical Artery (UA) Impedance Indices

Ratios calculated by measuring the blood flow waveforms from the uterine arteries through a free-floating portion of the umbilical cord.

The figure at the right shows the normal saw-tooth appearance of arterial blood flow through the umbilical cord.

S = Systolic peak (max velocity); The maximum velocity during contraction of the fetal heart.

D = End-diastolic flow; Continuing forward flow in the umbilical artery during the relaxation phase of the heartbeat.

Vm = Mean velocity

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.

SEE 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.


Umbilical cord

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 fetus.

Umbilical vein

The blood vessel that carries oxygen enriched blood from the placenta to the fetus.

Uniparental disomy

Both members of a chromosome pair are inherited from one parent rather than one from each parent. Uniparental disomy may be maternal or paternal.

Uterine contractions

Recurrent tightening and relaxation of the uterine wall (myometrium).

Uterine incision

A cut made into the wall of the uterus.

Uterine rupture

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.

Uterus (womb)

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 (Fallopian) tubes.

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' .  


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