The lower uterine segment is the thin, narrow portion of the uterus that sits between the main body (corpus) and the cervix. Outside of pregnancy, this area is called the isthmus and measures only about a centimeter in length. During pregnancy, it stretches and thins dramatically to become a distinct segment of the uterus, and it plays a central role in both labor and cesarean delivery.
Where It Sits in the Uterus
The uterus has four regions stacked from top to bottom: the fundus (the broad, curved top where the fallopian tubes connect), the corpus (the main body), the isthmus (the narrowed neck), and the cervix (the opening that projects into the vagina). The lower uterine segment corresponds to the isthmus, but it only takes on real clinical importance once pregnancy causes it to expand.
In a non-pregnant uterus, the isthmus is a short, barely noticeable transition zone. As the uterus grows during pregnancy, this zone unfolds and stretches into a wider, thinner-walled segment. By the third trimester, it forms the lower boundary of the space the baby occupies and sits just above the cervix. Because the muscle here is thinner than the upper uterus, it behaves differently during contractions and becomes the standard site for surgical delivery.
How It Changes During Pregnancy
The lower uterine segment thins progressively as pregnancy advances. Ultrasound measurements show the average thickness drops from roughly 6.9 mm around 12 weeks to about 4.4 mm by 22 weeks, then to 2.8 mm around 34 weeks, and finally 2.6 mm near 38 weeks. The thinning happens fastest during the first and second trimesters, then slows in the third trimester, reaching its thinnest point about one week before delivery.
This thinning is normal. It happens because the growing baby and increasing volume of amniotic fluid stretch the uterine wall, and the lower segment, with less muscular bulk than the upper uterus, stretches the most. During labor, contractions from the thicker upper segment pull upward on the lower segment, thinning it even further and helping draw the cervix open.
Why It Matters for Cesarean Delivery
When surgeons perform a cesarean section, the incision is almost always made horizontally across the lower uterine segment. This is called a lower-segment transverse incision, and it has become the standard approach for several reasons. The muscle wall here is thinner, which means less tissue to cut through and less blood loss. The thinner tissue also heals with a stronger scar compared to an incision through the thick, muscular upper uterus. That stronger scar is what makes it possible for many people to attempt vaginal birth in a future pregnancy.
The exact placement of this incision requires care. If the cut is made too low, it risks damaging the uterine artery, uterine vein, and ureter, all of which run close to the outer edges of the lower segment near the cervix. If labor has already progressed and the cervix is significantly dilated, the lower segment stretches upward, changing where the incision needs to go. Surgeons adjust the incision site based on how far labor has advanced to avoid cutting too close to either the cervix or the thicker upper uterus.
Thickness and Risk of Uterine Rupture
For anyone who has had a previous cesarean and wants to attempt vaginal birth next time (known as TOLAC, or trial of labor after cesarean), the thickness of the lower uterine segment is one of the key safety indicators. A previous cesarean leaves a scar in this segment, and if the wall becomes too thin, the scar can give way during labor contractions.
Clinicians use ultrasound to measure the segment’s thickness in the third trimester and classify rupture risk into three categories: low risk at 2.5 mm or above, moderate risk between 2.0 and 2.4 mm, and high risk below 2.0 mm. More recent research from the PRISMA trial found that all cases of uterine rupture during labor occurred in women whose lower segment measured below 3.0 mm when assessed with combined ultrasound approaches. None of the nearly 1,000 women with measurements at or above 3.0 mm experienced rupture. Based on these findings, a thickness below 3.0 mm is increasingly considered a meaningful risk factor.
Measurements can be taken through the abdomen (transabdominal ultrasound) or through the vagina (transvaginal ultrasound). Combining both approaches gives the most accurate prediction. Your care team uses these numbers alongside other factors, such as the type of previous uterine incision and how labor is progressing, to guide decisions about whether vaginal birth is safe to continue.
Placenta Previa and the Lower Segment
The lower uterine segment also becomes clinically important when the placenta attaches there instead of higher up on the uterine wall. This condition is called placenta previa, and it means the placenta partially or completely covers the cervix. Because the lower segment thins and stretches as pregnancy progresses, a placenta anchored there can separate from the wall, causing painless but sometimes severe bright red bleeding, typically after 20 weeks.
In some cases, an early ultrasound shows the placenta near the cervix, but it appears to “move” higher as the uterus grows and the lower segment stretches. If the placenta still covers the cervix later in pregnancy, delivery by cesarean section is necessary to avoid life-threatening bleeding. Placenta previa also raises the risk of a related group of conditions where the placenta grows abnormally deep into the uterine wall, which can cause significant bleeding during or after delivery.
How It Differs From the Upper Uterus
The upper uterus (the fundus and corpus) is thick, muscular, and does the heavy lifting during labor. Its contractions push the baby downward. The lower uterine segment, by contrast, is passive. It stretches and thins rather than contracting, creating a pathway for the baby to move through. This difference in behavior is what makes the lower segment ideal for surgical incisions: it’s under less mechanical stress during healing, and its thinner muscle produces a scar that’s less likely to rupture in future pregnancies compared to a scar in the actively contracting upper wall.
The two regions also drain to different lymph node groups, which can matter if uterine cancer is being staged. The lower segment drains toward the iliac lymph nodes along the major pelvic blood vessels, while the upper uterus drains along different pathways. For most people, though, the practical distinction comes down to pregnancy and delivery: the lower segment is where your body naturally creates the exit path for birth, and where surgeons place their incision when a cesarean is needed.