Background: Abnormal presentation (detected in the early third trimester) causes concern for pregnant women and their carers. Definitive ultrasound-based data on the risk of persistence of abnormal presentation is lacking to allow appropriate counselling. Comparison of pregnancy outcome was made on the basis of maternal age at delivery.
Methods: Notes of 1010 women (426 primigravidae, 584 multigravidae), with singleton pregnancies, confined between 1997 and 2005 were reviewed to extract: (i) the gestation based on 18–20-week ultrasound in conjunction with the patient's recorded last menstrual period, and (ii) the presentation of each antenatal visit from 28+ weeks until delivery. Previous obstetric history, maternal age, mode of delivery, birthweight and outcomes were also documented.
Results: At 28–30 weeks, 216 babies presented abnormally. By 38+ weeks, 54 persisted as either a breech or a transverse lie. Thus, an abnormal presentation in the early trimester carries a 22.2% chance of persisting at term. Continuance of abnormal presentation at each subsequent week of the third trimester increased the risk of a Caesarean delivery at term. Conversely, in only six cases, a cephalic presentation at 28–30 weeks converted to a breech or other presentation during the third trimester – a risk of 0.75%.
Conclusion: These statistics provide a useful tool in advising women of the chances of abnormal presentation at term based on the presentation at various stages of the third trimester, and prepare them for the potential requirement of a Caesarean section.