Perhaps the importance of the decision explains the length of time between the hearings (July 2014) and the judgment (11 March 2015) but there may be other reasons of which I am unaware.
Mrs Montgomery was of small stature and suffered from insulin dependent diabetes mellitus. Women suffering from diabetes are likely to have larger than normal babies. She underwent intensive monitoring during her pregnancy and was under the care of a Dr McLennan.
A particular risk was of shoulder dystocia which, should it occur, is a real obstetric emergency. Not only does it give rise to a risk of brachial plexus injury, but if the baby cannot be delivered promptly there is risk of serious oxygen deprivation, brain damage and even death of the child. The Supreme Court proceeded on evidence that there was about a 10% risk of shoulder dystocia occurring during the delivery of the child. Mrs Montgomery was not warned of that risk.
The leading judgment was given by Lord Kerr and Lord Reed, with whom four other Justice agreed, Lady Hale giving an additional but fully supportive judgment. Lord Kerr and Lord Reed noted at  :
"Unsurprisingly, Dr McLellan accepted that this was a high risk. But, despite the risk, she said that her practice was not to spend a lot of time, or indeed any time at all, discussing potential risks of shoulder dystocia. She explained that this was because, in her estimation, the risk of a grave problem for the baby resulting from shoulder dystocia was very small. She considered, therefore, that if the condition was mentioned, “most women will actually say, ‘I’d rather have a caesarean section’”. She went on to say that “if you were to mention shoulder dystocia to every [diabetic] patient, if you were to mention to any mother who faces labour that there is a very small risk of the baby dying in labour, then everyone would ask for a caesarean section, and it’s not in the maternal interests for women to have caesarean sections”."
Dr MacLennan accepted that Mrs Montgomery was concerned about the size of her baby but stated that she had never specifically asked about shoulder dystocia. The court noted that the risk of brachial plexus injury was 0.2% and the risk of prolonged hypoxia causing cerebral palsy or death was as low as 0.1%. There were also risks to the mother, including increased risk of post partum haemorrhage.
At delivery shoulder dystocia occurred and delivery took 12 minutes to accomplish (vividly described in the judgment) during which time the umbilical cord was wrapped around the baby's neck causing hypoxia and severe brain damage.
At first instance, and at first appeal, the courts rejected the claimant's claim on the basis - which was accepted by the Supreme Court - that a reasonable body of medical opinion would not have advised Mrs Montgomery of the risk of dystocia (and that it was not irrational to fail to do so). The lower courts considered that the case of Sidaway-v-Board of Governers ...  AC 891 remained good law and that it determined that the Bolam test applied to the issue of consent - was Dr MacLennan's conduct in accordance with a reasonable or responsible body of medical opinion.
Lord Kerr and Lord Reed noted and analysed the range of opinion stated by their Lordships in Sidaway. There was no unanimity as to whether the Bolam test should apply, unqualified or at all, to the question of consent. The application of Bolam to consent was further questioned in Pearce-v-United Bristol Healthcare NHS Trust 1999 PIQR P53 and Chester-v-Afshar.
In Pearce, a Court of Appeal decision, Lord Woolf MR said this:
“In a case where it is being alleged that a plaintiff has been deprived of the opportunity to make a proper decision as to what course he or she should take in relation to treatment, it seems to me to be the law, as indicated in the cases to which I have just referred, that if there is a significant risk which would affect the judgment of a reasonable patient, then in the normal course it is the responsibility of a doctor to inform the patient of that significant risk, if the information is needed so that the patient can determine for him or herself as to what course he or she should adopt.”
This then appeared to displace the reasonable doctor test with a prudent patient test. But Sidaway was a House of Lords decision.
Any lack of clarity has now been resolved. Lord Kerr and Lord Reed noted comparative jurisprudence including the Australian case of Rogers in which the court had noted that a doctor should be alert to the particular concerns of a patient about certain risks, which might be different from the concerns of a hypothetical reasonable patient. Lord Kerr and Lord Reed observed that there had been many social or societal developments and changes even since Sidaway. He said at  that what social and legal developments
"... point towards is an approach to the law which, instead of treating patients as placing themselves in the hands of their doctors (and then being prone to sue their doctors in the event of a disappointing outcome), treats them so far as possible as adults who are capable of understanding that medical treatment is uncertain of success and may involve risks, accepting responsibility for the taking of risks affecting their own lives, and living with the consequences of their choices."
And at  and :
"82. In the law of negligence, this approach entails a duty on the part of doctors to take reasonable care to ensure that a patient is aware of material risks of injury that are inherent in treatment. This can be understood, within the traditional framework of negligence, as a duty of care to avoid exposing a
person to a risk of injury which she would otherwise have avoided, but it is also the counterpart of the patient’s entitlement to decide whether or not to incur that risk. The existence of that entitlement, and the fact that its exercise does not depend exclusively on medical considerations, are important. They point to a fundamental distinction between, on the one hand, the doctor’s role when considering possible investigatory or treatment options and, on the other, her role in discussing with the patient any recommended treatment and possible alternatives, and the risks of injury which may be involved.
"83. The former role is an exercise of professional skill and judgment: what risks of injury are involved in an operation, for example, is a matter falling within the expertise of members of the medical profession. But it is a non sequitur to conclude that the question whether a risk of injury, or the availability of an alternative form of treatment, ought to be discussed with the patient is also a matter of purely professional judgment. The doctor’s advisory role cannot be regarded as solely an exercise of medical skill without leaving out of account the patient’s entitlement to decide on the risks to her health which she is willing to run (a decision which may be influenced by non-medical considerations). Responsibility for determining the nature and extent of a person’s rights rests with the courts, not with the medical professions."
And so at  Lord Kerr and Lord Reed set out the authoritative position on consent to medical treatment:
"The correct position, in relation to the risks of injury involved in treatment, can now be seen to be substantially that adopted in Sidaway by Lord Scarman, and by Lord Woolf MR in Pearce, subject to the refinement made by the High Court of Australia in Rogers-v-Whitaker which we have discussed at paras 77-73. An adult person of sound mind is entitled to decide which, if any, of the available forms of treatment to undergo, and her consent must be obtained before treatment interfering with her bodily integrity is undertaken. The doctor is therefore under a duty to take reasonable care to ensure that the patient is aware of any material risks involved in any recommended treatment, and of any reasonable alternative or variant treatments. The test of materiality is whether, in the circumstances of the particular case, a reasonable person in the patient’s position would be likely to attach significance to the risk, or the doctor is or should reasonably be aware that the particular patient would be likely to attach significance to it."
There is the therapeutic exception to this rule, namely that a doctor may withhold information from a patient if he considered it would be seriously detrimental to their health. Clearly that was not thought to apply in the present case (even though Dr MacLennan's view was that imparting information about risk of shoulder dystocia, and the possible complications from it, would lead all diabetic mothers to have Caesarean sections). Indeed Lord Kerr and Lord Reed warned that the therapeutic exception must not be abused.
They also noted that the assessment of risk could not be reduced to a question of percentages and, further, that the consent process involved a dialogue between doctor and patient.
The second part of the lead judgment concerns the issue of causation - an important issue but it was decided on its facts in the particular case. Of note, the Supreme Court were concerned to stress that the issue was not the risk of the complications of shoulder dystocia, but the risk of shoulder dystocia (about 10%) and of the various complications and procedures that SD might entail.
The Supreme Court does seem to have regarded elective Caesarean section as having virtually no risk at all . Perhaps this decision will lead, as Dr MacLennan thought likely, to Caesarean sections becoming the norm for diabetic mothers. If we live in a risk averse age - or at a time when we are still on the learner slopes when it comes to how best to weigh competing risks - then the removal of medical paternalism will be likely to affect the treatment choices patients make, perhaps leading to greater demand on NHS resources.
In reality, of course, it remains true that the manner in which a doctor gives information about risk can strongly influence what decision a patient makes. Many patients are content to be guided by their doctors. Other patients will have strong opinions about their treatment and will make decisions that their doctor thinks unwise. So be it - that is their right. Patient autonomy has to respected and that is now, unequivocally, the view of the Supreme Court.
On the other hand a patient does not have a right to demand that a doctor gives them whatever treatment they ask for. There are funding restrictions within the NHS and, in some cases, doctors may refuse to give treatment because they they believe that to do so would cause harm and would not be in the patient's best interests. Patient autonomy is not the same as a right to treatment on demand. Doctors have professional obligations.
The ramifications of this decision will require more reflection and consideration, but there can be no doubt that it is an important judgment that will affect clinical negligence litigation for years to come.