A baby is in a breech presentation when its bottom or feet are positioned to enter the birth canal first, rather than the head. This orientation contrasts with the typical head-down position, known as vertex presentation, which is generally achieved by most fetuses late in pregnancy. Concern for breech usually begins around 32 to 36 weeks, as the fetus has grown significantly and space is becoming limited. There are different types of breech, including frank breech (hips flexed, legs straight up), complete breech (hips and knees flexed), and footling or incomplete breech (one or both feet positioned to lead the way out).
Fetal Factors Preventing Rotation
A significant portion of breech presentations is attributed to factors intrinsic to the fetus, which inhibit the natural turning process. Prematurity is the most common factor, as spontaneous rotation to the head-down position often occurs between 32 and 36 weeks of gestation. Fetuses born before this time have not yet completed the normal developmental maneuver that moves their heavier head toward the pelvis. At 28 weeks, about 25% of fetuses are still in a breech position, which drops to only 3% to 4% by the time a pregnancy reaches term.
Multiple gestation, such as twins or triplets, significantly reduces the physical space available for each fetus to maneuver and settle into the head-down position. The presence of multiple babies means they compete for room within the uterine cavity, often resulting in one or more remaining in a breech or transverse lie. This crowding effect prevents the necessary somersaulting movement.
Certain fetal anomalies can prevent the baby from rotating or maintaining a head-down position due to issues with movement or head size. Conditions affecting the central nervous system or musculoskeletal structure can inhibit the coordinated muscle movements needed for complex rotation. Hydrocephaly, which involves an enlarged head, can also make it mechanically more difficult for the head to enter the narrow lower segment of the uterus. Similarly, conditions like neck masses can interfere with the flexion and tucking of the head required to move into the vertex position.
Uterine Structure and Placement Issues
The shape and structure of the uterus, along with the location of the placenta, can physically impede the fetus from turning head-down. Uterine malformations, resulting from congenital differences in the mother’s anatomy, can create an irregular or divided uterine cavity. For example, a bicornuate or septate uterus features a partial or full division, which effectively limits the available space, making it impossible for the baby to complete the rotation.
The presence of uterine fibroids, which are non-cancerous growths, can occupy space and act as a physical barrier within the uterus. Depending on their size and location, these tumors can prevent the fetus from having enough room to flip or can block the lower uterine segment where the head would normally descend. The fixed position of the placenta can also contribute to a breech presentation when it is located low in the uterus.
Placenta previa is a condition where the placenta partially or completely covers the cervix, blocking the natural exit route. This low-lying placental placement physically obstructs the baby’s head from descending into the pelvis, which is necessary for final head-down engagement. This structural obstruction forces the heavier fetal head to remain in the upper part of the uterus, fixing the baby in a breech position.
Restricted Mobility from Fluid and Cord
The immediate environment surrounding the fetus, specifically the volume of amniotic fluid and the condition of the umbilical cord, plays a significant role in fetal mobility. Oligohydramnios, characterized by abnormally low levels of amniotic fluid, is a major factor in fixing a baby in a breech position. The fluid acts as a cushion and lubricant, and its severe reduction removes the necessary space and buoyancy the fetus needs to execute the somersaulting movement.
Conversely, polyhydramnios, an excess of amniotic fluid, can contribute to a persistent breech presentation through a different mechanism. While the abundance of fluid allows the baby to float and move freely, it can prevent the fetus from stabilizing in the preferred head-down position. The baby may continuously flip back and forth, failing to settle and engage its head in the pelvis.
The umbilical cord can act as a physical constraint on fetal movement. Research suggests that the umbilical cords of babies remaining breech at term are often significantly shorter than those who are head-down (average length is 55 to 60 centimeters). A cord that is inherently short, or effectively shortened by being wrapped around the baby, physically tethers the fetus, limiting the range of motion required for a full rotation.