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What is Cephalic presentation and explain in detail?
A cephalic presentation or head presentation or head-first presentation is a situation at childbirth where the fetus is in a longitudinal lie and  the  head enters the pelvis first;  the most common form of cephalic presentation is the vertex presentation, where the occiput is the leading part (the part that first enters the birth canal). All other presentations are abnormal (malpresentations) and are either more difficult to deliver or not deliverable by natural means.

Engagement
The movement of the fetus to cephalic presentation is called head engagement. It occurs in the third trimester. In head engagement, the fetal head descends into the pelvic cavity so that only a small part (or none) of it can be felt abdominally. The perineum and cervix are further flattened and the head may be felt vaginally. Head engagement is known colloquially as the baby drop, and in natural medicine as the lightening because of the release of pressure on the upper abdomen and renewed ease in breathing. However, it severely reduces bladder capacity resulting in a need to void more frequently.

Classification
In the vertex presentation the head is flexed and the occiput leads the way. This is the most common configuration and seen at term in 95% of singletons. If the head is extended, the face becomes the leading part. Face presentations account for less than 1% of presentations at term. In the sinicipital presentation the large fontanelle is the presenting part; with further labor the head will either flex or extend more so that in the end this presentation leads to a vertex or face presentation. In the brow presentation the head is slightly extended, but less than in the face presentation. The chin presentation is a variant of the face presentation with maximum extension of the head.
Non-cephalic presentations are the breech presentation (3.5%) and the shoulder presentation (0.5%).

Vertex presentation
The vertex is the area of the vault bounded anteriorly by the anterior fontanelle and the coronal suture, posteriorly by the posterior fontanelle and the lambdoid suture and laterally by 2 lines passing through the parietal eminences.
In the vertex presentation the occiput typically is anterior and thus in an optimal position to negotiate the pelvic curve by extending the head. In an occiput posterior position, labor becomes prolonged, and more operative interventions are deemed necessary. The prevalence of the persistent occiput posterior is given as 4.7%.The vertex presentations are further classified according to the position of the occiput, both right, left, or transverse and anterior or posterior:

Left Occipito-Anterior (LOA), Left Occipito-Posterior (LOP), Left Occipito-Transverse (LOT)
Right Occipito-Anterior (ROA), Right Occipito-Posterior (ROP), Right Occipito-Transverse (ROT)The Occipito-Anterior position is ideal for birth; it means that the baby is lined up so as to fit through the pelvis as easily as possible. The baby is head down, facing the spine, with its back anterior. In this position, the babys chin is tucked onto its chest, so that the smallest part of its head will be applied to the cervix first. The position is usually "Left Occiput Anterior", or LOA. Occasionally, the baby may be "Right Occiput Anterior", or ROA.

Face presentation
Factors that predispose to face presentation are prematurity, macrosomia, anencephaly and other malformations, cephalopelvic disproportion, and polyhydramnios. In an uncomplicated face presentation duration of labor is not altered. Perinatal losses with face presentation occur with traumatic version and extraction and midforceps procedures Duff indicates that the prevalence of face presentations is about 1/500–600., while Benedetti et al. found it to be 1/1,250 term deliveries.Face presentations are classified according to the position of the chin (mentum):

Left Mento-Anterior (LMA), Left Mento-Posterior (LMP), Left Mento-Transverse (LMT)
Right Mento-Anterior (RMA), Right Mento-Posterior (RMP), Right Mento-Transverse (RMT)

Brow presentation
While some consider the brow presentation as an intermediate stage towards the face presentation, others disagree. Thus Bhal et al. indicated that both conditions are about equally common (1/994 face and 1/755 brow positions), and that prematurity was more common with face while postmaturity was more common with brow positions.

Oskie presentation
The Oskie presentation is similar to the Occipito-Anterior position, where the baby is head down, facing the spine, with back on the ventral side of the uterus; however, in this position, while the torso is aligned with the mothers longitudinal axis, the legs of the fetus are extended straight along the frontal axis of the mother, as if the baby is creating a right angle with its body. For the Oskie position to occur the babys head must be far down the pelvis in order to allow room for leg extension, typically the arms are bent, tucked against the babys body