Its hard to tell baby's head position from a simple vaginal exam. The baby's skull is made up of portions of the skull that move and overlap somewhat. These are called "plates" and they have names. The borders between them are mobile and are called "sutures." Sutures lines are supposed to reveal fetal head position. Is it worth an ultrasound in a difficult labor?

The angle of the head can put a suture or soft spot in an unexpect place or, often out of reach. A baby's skin makes wrinkles as the baby's head molds and can hide a portion of the suture or the soft spot. Sometimes, when space is an issue, the baby's head will swell where it presses down. These alterations can confuse a person. Provider's fingers come in different lengths and sensitivities. Skill varies. All in all, "digital" vaginal exams, meaning those with the digits or fingers, are not as accurate or as fast as a laboring woman would hope.

Babies turn their heads to get through the pelvis. Every once in a while, a baby will turn his or her head so far as to be in the opposite direction as the chest! I've seen it at least 3 times myself, and now that I get it, probably more in the past than I knew.

Before a doctor decides to put a vacuum on a baby to bring the head out of the pelvis or before the doctor or midwife (in unusual cases) decides to manually rotate the baby's head, it is advised to get an ultrasound to double check the baby's head position with a more accurate tool than touch alone. 

Minerva Ginecol. 2007 Aug;59(4):459-64.

Occipito-posterior fetal head position, maternal and neonatal outcome

Martino V, Iliceto N, Simeoni U.

Abstract

The purpose of this review is to summarize the available evidence on occipito-posterior fetal head position and maternal and neonatal outcome.

The occipito-posterior fetal head position is the most common malposition, but there are not so many data about it in literature. Its incidence is ranging from 1.8% by Fitzpatrick, to 4.6% and 5.5% by Yancey and Sizer, to 6% by Ponkey.

Only two trials studied the occipito-posterior associated factors. There are lower incidence of premature rupture of membrane, arterial hypertension pregnancy-induced, induced labour, increased of episiotomy, instrumental delivery and a decreased of vaginal birth without a difference in neonatal Apgar, and with a neonatal bigger weight. The occipito-posterior fetal head position persistence compared to anterior position, has a statistically significant association with low maternal stature, previous cesarean section, longer first and second stage of labour, oxytocin augmentation, epidural analgesia, instrumental vaginal delivery, chorion-amniositis, vaginal perineal injures, loss of blood and post partum infections. A highest incidence of occipito-posterior fetal head position may depend by nulliparity, malnutrition with pelvic deformity, pelvic immaturity in the teenager and anterior placenta. Epidural analgesia is a risk factor for fetal head malposition. The majority of occipito-posterior fetal head positions is not due to a malrotation, but to a persistence in this position of the fetal head. In fact, this persistence leads to a failure of the fetal head rotation. The prolonged second stage is often the result of occipito-posterior fetal head position and instrumental delivery is required.

The traditional vaginal examination is not useful for the determination of fetal head position, so and instrumental method is needed, such as ultrasound, for a correct evaluation of fetal head position, particularly if a vaginal instrumental delivery is necessary. This is recommended by the Canadian Society of Obstetrics and Gynecology. The evaluation of fetal head position is important in the prediction of labour induction.

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