Breech presentation occurs when the fetus is positioned with the buttocks, feet, or both aimed toward the mother’s pelvis, rather than the head. This differs from the usual head-down, or cephalic, presentation that most babies adopt in the final weeks of pregnancy. While the majority of fetuses are head-down by term, breech presentation occurs in approximately 3% to 4% of all full-term deliveries. Understanding the reasons for this variation involves examining the physical space within the uterus, the baby’s developmental status, and the mother’s anatomical history.
How Uterine Environment Influences Position
The physical conditions inside the uterus significantly determine if a fetus can settle into the head-down position. The volume of amniotic fluid directly affects fetal mobility. Excessive fluid (polyhydramnios) gives the baby too much room, allowing it to flip easily without settling late in the third trimester. Conversely, a severely low volume (oligohydramnios) restricts movement, making it physically difficult to execute the necessary rotation into the cephalic presentation. Both extremes impede the natural turning process.
The location of the placenta can also physically obstruct the baby’s path. When the placenta partially or completely covers the cervix opening (placenta previa), it occupies the space where the baby’s head would normally descend. This barrier prevents the presenting part from engaging in the pelvis, encouraging a breech orientation. Furthermore, structural anomalies of the uterus, such as a bicornuate or septate uterus, limit internal space. These variations in uterine shape can physically crowd the fetus and hinder the rotation necessary for a head-first presentation.
Factors Related to Fetal Condition and Development
Gestational age is a strong predictor of breech presentation, as the baby typically settles head-down only in the later stages of the third trimester. For example, at 32 weeks, about 7% of fetuses may be breech, but this percentage drops significantly to the term rate of 3% to 4%. Many earlier breech presentations are temporary phases before spontaneous version occurs. Premature birth is correlated with breech presentation because the baby is delivered before this natural turning process is complete.
Multiple gestations, such as twins or triplets, often result in one or more babies being breech due to shared space limitations. The crowded environment restricts the ability of each fetus to fully extend and rotate into the optimal head-down position. The available space may not accommodate multiple babies in the most efficient alignment.
Fetal Anomalies
Certain conditions related to fetal development can affect the baby’s ability to turn or maintain the head-down position. This includes fetal anomalies affecting the central nervous system or neuromuscular function, which can cause hypotonia (low muscle tone). Reduced muscle strength can prevent the baby from actively initiating or sustaining the movements needed to rotate. Structural issues, such as an enlarged head size or masses on the neck, can also mechanically impede the ability to turn or fit into the lower segment of the uterus.
Maternal History and Structural Constraints
The mother’s reproductive system structure and previous obstetric history contribute to the likelihood of a breech presentation. Benign muscular tumors called uterine fibroids can grow within the uterine walls, creating an internal obstruction. Depending on their size and location, fibroids can physically block the baby’s path or alter the uterine cavity shape, preventing the fetus from settling head-down.
A woman’s obstetric history, including parity, is also a factor. First-time mothers (nulliparous women) have a slightly higher risk of breech presentation at term compared to those with previous pregnancies. This difference relates to the firmness of the uterine muscles and abdominal wall, which may be more restrictive in a first pregnancy. If a woman has had a previous breech delivery, the likelihood of a subsequent breech presentation increases.
Scarring from past uterine surgery, including prior Cesarean sections, can affect the integrity and shape of the uterus. Scars may alter the flexibility of the uterine wall, influencing how the baby positions itself during the final trimester. An unusually shaped or contracted maternal pelvis may also make it less favorable for the baby’s head to descend and engage. The pelvic structure can make the head-down position physically uncomfortable or impossible for the fetus to maintain.