3. Placenta previa:

This topic is also covered in MCQ5, question 24.

'Accidental' haemorrhage is the old term for bleeding from placental abruption.

In this, the placenta separates prematurely – see MCQ9, question 31. 

Placenta previa is where some or all of the placenta is in the lower segment.

It was graded I - IV, but is now subdivided into “major” and “minor”. 

“Major” is when it reaches the cervical os, “minor” when it doesn’t. 

Major placenta previa has a significant risk of APH and obligatory Caesarean section.

Minor placenta previa has less risk of bleeding and the patient may deliver vaginally. 

If the placental edge is within 2 cm. of the os, Caesarean section will usually be necessary. 

On the other hand, if the edge is > 4.5 cm. from the os, Caesarean section is unlikely. 

If the placenta is thick or posterior, the risk of Caesarean section increases. 

The risk of placenta previa increases with maternal age and uterine surgery:

    e.g. D&C, myomectomy and, particularly, Caesarean section, so the incidence is increasing. 

The risk of abnormal placental adherence is also linked to previous Caesarean section.

So think of abnormal adherence if the placenta is anterior and the woman previously had Caesarean section.

About 5% of patients will have a 'low' placenta on the 18 -19 week scan on abdominal scanning. 

Only about one in ten of them will be seen to have placenta previa in the third trimester.

By this time the lower segment has formed.

So the incidence is 0.5%. 

Transvaginal scanning (TVS) is much more accurate than abdominal scanning at 20 weeks and does not provoke bleeding. 

The College guideline (October 05) recommends that the ‘low’ placenta at 20 weeks should be investigated with TVS. 

This will put up to 60% of ‘low’ placentas into a normal category. 

A repeat scan later in pregnancy is recommended for the rest; at 32 weeks for the “major” ones and 36 weeks for the “minor”. 

This will show many not to be low at all, even some of those that were covering the os, and is due to placental “migration”. 

This is a bit of a misnomer. 

The placenta does not shuffle back up the lower segment. 

The lower segment develops from little more than a line at 20 weeks to being several centimetres wide at 36 weeks.

It is the bit we go through to do a lower segment Caesarean section.

The placenta that appeared to reach the os at 20 weeks, might be several cms. away by the time the lower segment is fully formed. 

Caesarean section has traditionally been done under general anaesthesia, but spinal blocks are increasingly being used. 

Senior obstetric and anaesthetic staff  need to be present. 

This is especially important if the patient has previously had Caesarean section.

In these cases the main risk is placenta accreta, but you could also have adhesions or an abnormally adherent bladder.

Most of those suspected of having placenta previa at 20 weeks will have no problem.

But they should still be given practical advice about their increased risk of bleeding. 

The risk is small under 26 weeks, but patients should avoid putting themselves in situations where bleeding would be hazardous. 

This could mean restrictions on flying or changing the nature of a job: the sales rep being office-based rather than out on the road. 

A mobile phone is a boon. 

She must be advised to get to hospital straight away if she has bleeding, pain or ruptured membranes.

The management of bleeding is conservative.

The aim is to prolong the pregnancy as far as possible up to 38 weeks to minimise the risk to the baby from prematurity. 

Tocolytics may be used.

But this remains a contentious area given the potential adverse side effects on the cardio-vascular system of the betamimetics such as Ritodrine.

However, it is fast becoming of historic interest as betamimetics go out of fashion.

The risk of postpartum haemorrhage may be related to the thin lower segment being less able to act as the “living ligature” (see question 8 below).

It could also be due to an increased placental bed area (see MCQ5, question 24).

Patients with major degrees of placenta previa, or who have bled, are safest in hospital. 

The College guideline advises admission at 34 weeks for those with “major” placenta previa who have had bleeding. 

Cross-matched blood may need to be available. This should be discussed with the blood bank. 

As most of this blood will not be used, it is an awful waste, on the other hand it could be life-saving. 

Patients with unusual blood groups or antibodies particularly need to be discussed with the laboratory. 

What to do with the “major” cases who have not bled is more difficult. 

They can be at home if they have someone with them all the time, a phone and facilities for quick transport to the hospital. 

However, there are still risks and these must be explained in detail, accepted and the discussion documented. 

Patients with minor degrees and no bleeding may be managed as outpatients so long as they can get to hospital rapidly in the event of bleeding. 

All patients need to be counselled about the risks of complications:

    bleeding,

    need for blood transfusion,

    premature delivery,

    Caesarean section,

    PPH,

    hysterectomy etc. 

Hospitalised patients are at increased risk of thromboembolism and each patient should be assessed in this regard.

I do not know why the incidence of fetal abnormality doubles.

You should also exclude placenta previa if a patient has a high presenting part or an abnormal or unstable lie in late pregnancy.