Birth anatomy

Before birth

Essentially, before the birth occurs, there are three things to thing about. Helpfully, they all begin with 'P' (or at least their synonyms do). These are the passage, passenger and power and there are a variety of physiological changes to each.

Passage

Cross-section of the pelvis with some terms often used in obstetrics

The diagram on the right shows the pelvis in cross-section. The measurements mentioned at each level are AP diameter (anterior-posterior diameter - front-to-back distance) and transverse diameter (side-to-side diameter). Obviously, baby's head has to be able to get through these. Essentially, AP x transverse:

• Inlet - 11 x 13
• Mid-cavity - AP=transverse
• Outlet - 12.5 x 11

The cervix, vagina and perineum all need to be overcome during delivery but this will be covered later on.

Passenger

Top of foetal skull, showing anterior (bregma) and posterior (occiput) fontanelles

On the left is a diagram of the top of the foetus skull. The reason for this diagram is that these sutures and fontanelles are vaginally palpable. Essentially, this allows you to figure out the presentation of the foetus and some presentations are abnormal. There are two things that need to be determined on palpation of the fetal skull at delivery:

• Attitude - this is basically how much the neck is flexed. The ideal attitude is for the neck to be well-flexed. The head can be deflexed, extended 90^o (brow presentation) and hyperextended to 120^o (face presentation).
• Position - ideally, when the baby's head enters the inlet of the pelvis it should be in the occipto-transverse (OT) position (the sagittal suture should be transverse). However, when the babies head reaches the outlet it needs to be vertical, i.e. occipito-anterior (OA) position. Hence the head rotates 90^o during its transit through the vagina. Approximately 5% of deliveries are, you guessed it, occipito-posterior (OP) when they reach the outlet which makes delivery more complicated. If the OT position persists, normal delivery will not be possible.
• Size of head - the head is usually compressed as it goes through the pelvis (called moulding). If a swelling is caused during delivery, it is called a caput.

Power

This basically means the women do not have the neccessary power needed to push a baby out. It is rarely a problem in multiparous (previously given birth) women but is common in nulliparous (never given birth) women. This may be aided by instrumental delivery.