Fetal size at the 32nd gestational week is associated with the risk of breech presentation at term birth

The association patterns between breech presentation at birth and fetal biometry at the first, second, and third trimesters, newborn size but also maternal age, body height, prepregnancy weight status as well as gestational weight gain, were analyzed using a dataset of 4501 singleton term birth in Vienna, Austria.

In this medical record-based study, fetal biometry was reconstructed based on the results of three ultrasound examinations conducted at the 11th/12th, 20th, and 32nd gestational weeks. Head dimensions, abdominal dimensions, and femur length were determined by sonography. Birth weight, birth length, and head circumference were measured immediately after birth.

The total breech presentation rate at birth was 6.2%. Breech newborns were significantly (p < 0.001) shorter and lighter at the time of birth, their head circumferences, however, were significantly larger (p = 0.001). At the 32nd week, breech fetuses showed significantly smaller biparietal breadths, but highly significantly longer heads. Their abdominal dimensions were significantly smaller, and their femora were shorter. Higher maternal age, and a longer, but narrower fetal head as well as smaller abdominal dimensions at the 32nd gestational week were independently related to a higher risk of breech presentation at the time of birth.

Fetuses who remain in a breech presentation until term birth (≥37 gestational weeks) differed significantly in head and abdominal dimensions from cephalic fetuses from the 32nd gestational week onwards.