6.       Erect lateral pelvimetry:     

Key facts for the DRCOG.

X-ray pelvimetry is rarely used these days and many obstetricians do not believe in it at all.

They feel that the information gained is not worth exposing the woman to radiation.

CT and MR imaging have been used, but have not been proven to be more useful.

Probably the only use would be after pelvic fracture.

But even then the x-rays taken at the time of the injury would probably suffice.

Pelvimetry might be considered if unsuspected cephalo-pelvic disproportion could carry an unusually high cost.

This would be in breech presentation, where the head might get stuck.

And vaginal birth after Caesarean section (VBAC), where the uterus might rupture, (see MCQ12, question 16).

However, even in these situations x-ray pelvimetry is not recommended.

As far as the DRCOG is concerned x-ray pelvimetry is passé.

Expanded information for the MRCOG and to help facts stick.

The following gives a more detailed explanation, of value to MRCOG candidates, and some other stuff, like the benefits of flexion.

The College guideline on breech delivery (December 2006) says that:

    clinical assessment of the pelvis has traditionally been regarded as adequate,

    and that studies of x-ray pelvimetry have not shown benefit.

It mentions that the “Term Breech Trial” on which current management of breech delivery is largely based.

This also found no benefit from x-ray pelvimetry.

The guideline mentions that one trial showed fewer emergency Caesarean sections after MR pelvimetry.

But no improvement to perinatal outcome or the overall number of Caesarean sections.

It also mentions one study (retrospective) suggesting that CT pelvimetry might improve perinatal outcome.

It has been taught for years that x-ray pelvimetry did not help in the assessment of the woman planning VBAC.

The recent College guideline on VBAC (February 2007) does not even mention pelvimetry.

Pelvimetry shows the size and shape of the pelvis.

But the passage of a baby through the pelvis is a dynamic process, relating to:

    strength of contractions,

    position of the baby's head e.g. occipito-posterior,

    degree of flexion etc.

If a pelvis is extremely small, most babies will not pass through.

However, even with a pelvis of good size, factors such as the presentation and degree of flexion are important.

Brow presentations, mento-posterior face presentations and deflexed OP positions present large diameters to the pelvis.

To explain the importance of flexion, I usually ask students to form a circle with thumb and ring finger.

And then imagine how they would pass a hen’s egg through the hole.

They will conclude that the egg will need to be "end on" and that “sideways on” it will not go through.

A baby’s head is similar.

The best diameters come with the baby’s chin tucked down on its chest so that the head is flexed and the vertex leads.

Ask a mother how she will put a tight jumper over a baby’s head.

She will tell you that she will apply the neck opening to the baby’s occiput and then pull the jumper on.

Without knowing it, she is making use of the smallest diameters.

The more the head deflexes, the worse are the diameters presenting to the pelvis.

This culminates in the completely deflexed head (brow presentation), which will not deliver vaginally, even if the baby weighs only a few pounds.

Pelvimetry can be done immediately after delivery.

It is the renal examination that has to be delayed to allow the dilatation of the renal tract associated with pregnancy to abate.

Attempts have been made to improve old-fashioned x-ray pelvimetry using CT or MR scanning.

They are more accurate and reduce or eliminate the radiation dosage to the mother.

But there is no evidence that they are more helpful.

Probably because measuring the size and shape of the pelvis is inherently of little use.

Clinical pelvimetry used to be routine at 36 weeks.

This gave some vague idea of the size and shape of the pelvis.

It is almost always a waste of time and a horrible ordeal for the poor patient. (See also MCQ3, question 1.)

It was one of the first things I banned from my antenatal clinics.

If you encounter a Consultant still inflicting this nastiness on his patients, you could reasonably suggest that it is time that they retired!

It is important to stop and think before conducting any vaginal examination.

Most of my antenatal patients will get through the whole of pregnancy without one.

These only becoming necessary to assess the progress of labour.

Some have come to think that women are no longer bothered about vaginal examination.

When I encounter this view in a young doctor, I get them to ask patients in the antenatal clinic who need vaginal examination e.g. for assessment of suitability for induction of labour, if the patient would rather have a midwife or a male doctor do the examination.

Almost universally they prefer to have the midwife.

If there were some other test to spare them vaginal examination, they would opt for that as well.

It should join even more ancient practices such as reading the runes