Damage to the facial nerve (also called the seventh cranial nerve) just before or at the time of delivery may cause paralysis to the same side of the face stimulated by the damaged nerve. The entire side -- from the forehead to the chin -- may be involved in severe cases.

The most common form involves only the lower branch of the facial nerve, however. This nerve controls the facial muscles surrounding the lips, and the defect is seen predominantly when the infant cries.

Since the facial muscles on the affected side are weak, the mouth is pulled towards the normal side during crying. Generally, the nerve has been damaged by pressure and the nerve fiber itself has not been torn, in which case, the injury heals and the face returns to normal over a few weeks to months.

Most of the times, a congenital (present from birth) facial nerve palsy like this one has no apparent cause. Occasionally, however, a difficult delivery, with or without the use of forceps (instruments used by physicians to facilitate the delivery of the baby´s head), may result in facial paralysis.

Some factors are associated with birth trauma, including prolonged pregnancy, epidural anesthesia, prolonged labor, oxytocin use (a medication used to induce labor and increase the strength of contractions of the uterus during labor), and large baby size (for instance, when the mother suffers from diabetes). However, most mothers who have these factors do not deliver babies with facial palsy or birth trauma.

• Present immediately after birth
• Failure of the lower lip to pull down symmetrically while crying (typical)
• Failure of any movement on the affected side of the face (total involvement, unusual)
• Eyelid may not close on affected side
• Lower face (below eyes) appears asymmetrical while crying

Physical examination is usually sufficient to diagnose this condition. Rarely, a nerve conduction study with electromyogram (NCS/EMG) is needed. The NCS/EMG tests the electrical function of the nerve and can pinpoint where the nerve was injured.

Brain imaging techniques are deemed not necessary, unless a brain problem is suspected instead of a facial nerve injury.

This condition is generally diagnosed while the infant is in the hospital. Mild cases involving just the lower lip may escape observation and later be noticed by a parent, grandparent, or other visitor.

If your infant's mouth does not look symmetrical when he or she cries, you may want to call your health care provider to have the problem diagnosed and rule out other conditions.