Reproductive system · Pelvis · Female
Lower uterine segment !
The lower uterine segment is the thin lower part of the uterus in late pregnancy, just above the cervix; most caesarean incisions are made across it.
Official name: Isthmus of uterus
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What it is
The narrow part between the body and the cervix (isthmus), which thins and stretches in late pregnancy.
What it does
Most caesarean incisions are made across it because it bleeds less and heals stronger.
Key facts
- Where
- Isthmus of the uterus between the body and the cervix, at the level of the internal os; thins and lengthens in late pregnancy
- Blood supply
- Uterine artery (main trunk turns upward at this level)
- Nerve
- Uterovaginal plexus (from inferior hypogastric plexus): sympathetic T10–L1, parasympathetic S2–S4
- Function
- Site of the low transverse caesarean incision (less bleeding, stronger scar)
Relations
- In front
- Vesicouterine pouch and the urinary bladder; the peritoneum turns from the uterus onto the bladder at about this level
- Behind
- Rectouterine pouch
- Lateral
- The uterine artery reaches the uterus here and turns upward along its side, after crossing above the ureters
- Above
- Body of uterus
- Below
- Cervix, from the internal os
How to find it
Not palpable as such outside pregnancy; on ultrasound it is the short narrowing between the body of uterus and the cervix. At term, at caesarean section, it is the thin, pale part of the uterus beneath the loose vesicouterine peritoneum, above the bladder.
Examination
- Hegar sign: in early pregnancy the isthmus feels so soft on bimanual examination that the fingers of the two hands seem to meet through it
On imaging
After a caesarean, ultrasound can show a scar niche (isthmocele) and measure the thickness of the lower segment; in pregnancy it shows how far the placental edge lies from the internal os, and Doppler or MRI look for signs of placenta accreta spectrum.
Common problems
- Fibroids (benign muscle growths).
- Heavy or painful periods; adenomyosis.
- Endometrial cancer, usually after menopause.
- Prolapse (the uterus drops into the vagina).
How it is diagnosed
Pelvic examination and ultrasound (often transvaginal); hysteroscopy or a sample of the lining (biopsy) when needed; MRI for some cases.
Treatment
- The site of the uterine incision in a caesarean section.
Clinical notes
- Bandl ring (pathological retraction ring): in obstructed labour the upper segment thickens and the lower one thins too much; a ring rising on the abdomen warns of uterine rupture
- Caesarean scar pregnancy implants in the niche of an old scar, with a high risk of bleeding and rupture
- The uterine vessels lie close to the lateral ends of a low transverse incision; an incision that extends sideways can tear them
Development
It is formed from the isthmus of the non-pregnant uterus, which is drawn out and thinned from about the middle of pregnancy and most of all in labour; this is why it barely exists before term and why a caesarean before labour finds it thicker and shorter.
Related conditions
Test yourself
Think of the answer, then tap the question to see it.
How do the upper and lower uterine segments behave in labour?
The upper segment contracts and thickens; the lower uterine segment stretches and thins passively. The boundary is the physiological retraction ring.
Why can a trial of labour be offered after a low transverse caesarean scar?
A low transverse scar in the lower uterine segment ruptures far less often than a classical vertical upper-segment scar.
What does a Bandl ring rising on the abdomen warn of?
Impending uterine rupture in obstructed labour, as the lower uterine segment thins too much.
What is the Hegar sign?
In early pregnancy the isthmus (lower uterine segment) is so soft on bimanual examination that the fingers seem to meet through it.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- isthmus uteri TA2 3510
- Official Spanish (DeCS)
- Istmo del útero DeCS D014599
- English (TA2)
- Isthmus of uterus
- Also called
- orificium internum canalis isthmi
Related
Sources: Terminologia Anatomica (TA2); DeCS; Anatomy, Abdomen and Pelvis: Uterus.
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Representative model, for educational purposes only.
