The Department of Health has issued new guidelines for doctors and health professionals, who advise young people under 16 on contraception, sexual health and abortion.
Supporters say the guidance will help to protect girls, reduce teenage pregnancy and tackle the spread of infections.
But anti-abortion campaigners argue it ignores the parent's right to know what is best for their child.
Guidelines issued to teachers by the Department of Education (DfES) are not affected by the new advice.
Q: Is it legal for schools and doctors not to tell parents if a girl under 16 gets pregnant?
A: Yes, it is still legal.
However, the new guidelines say health professionals should stress the benefits of informing the girl's GP and encouraging discussion with a parent and carer.
If she refuses to involve a parent, she should be encouraged to talk to another adult such as a family member or a specialist youth worker.
For health professionals, confidentiality remains a duty, except when a young person's right to privacy is outweighed by a risk to her health, safety and welfare. In this case, safeguarding the young person becomes the overriding objective.
British teen pregnancy rates are high
Guidelines from The British Medical Association, which represents doctors, say: "The duty of confidentiality owed to a person under 16 is as great as that owed to any other person."
The law in Scotland and Northern Ireland is similar - although in Northern Ireland rules on abortion are different.
Q: What do teachers' guidelines say?
A: Under DfES guidelines, teachers are not legally bound to inform parents of any disclosure by pupils unless the school's confidentiality policy requires them to do so.
But nor do they have any specific duty of confidentiality to the child, which means that in some schools teachers will tell parents about a pregnancy even if the girl does not want them to.
However, "teachers should seek consent for any disclosure", the guidelines state.
But if a child goes first to a healthcare worker rather than a teacher, confidentiality will be more or less guaranteed.
Q: What happens in practice - are teenagers encouraged to tell their parents?
A: For teachers, the DfES guidelines state that schools should take steps to ensure that "wherever possible, the young person is persuaded to talk to their parent or carer".
BMA guidelines also state: "Doctors have an obligation... to encourage the patient voluntarily to involve parents".
This is important because of the vital role of family support in such potentially traumatic circumstances, the BMA says.
However, if the girl insists to a doctor or nurse that her parents are not told, they will not be.
Q: For doctors, what would be considered an exceptional circumstance in which confidentiality could be broken?
Almost all girls who fall pregnant would be deemed to have "sufficient understanding" of their situation to make their own choices of treatment, says the BMA.
The exceptions would be, for instance, someone with mental health difficulties.
As for "convincing reasons" to break confidentiality, they would also be extremely rare, such as suspected rape.
Even then, this may not mean parents or carers being involved. It could mean a doctor discussing the matter with another doctor, or asking the girl to consent to a social worker being brought in.
Q: Why is it so important that a young person's confidentiality be respected?
A: Confidentiality is considered an important weapon in the fight to bring down the UK's soaring teenage pregnancy rates.
Doctors, nurses and other healthcare professionals say it is vital that young people feel they can speak to them in secret.
Even as it is, they say, many young people fear their parents will be told if they ask about delicate subjects such as pregnancy, HIV and contraception - and could therefore be risking getting into more difficulties.
With a pregnancy, it is important that young people seek advice as soon as possible while there are still some choices left, they say.
Q: What are the rules on abortion?
A: In England, Wales and Scotland the 1967 Abortion Act means a woman of any age can have an abortion if she is less than 24 weeks (about five to six months) pregnant, if two doctors agree she has good reasons.
On abortion for under-16s, the BMA guidelines for England and Wales say: "As with other medical interventions, a person who has sufficient understanding... and is acting free from pressure may give valid consent to the termination of pregnancy regardless of age."
In Northern Ireland, abortion is usually illegal at any age, although an estimated 70 or so are carried out every year.
It is possible the Northern Ireland regulations could change - they were recently the subject of a failed judicial review which is now being appealed.
Q: But isn't it illegal to have sex under 16?
A: It is illegal in England, Scotland and Wales to have heterosexual sex under 16 - and in Northern Ireland under 17.
However, in practice prosecutions are rare as long as both people consented and there is no evidence of exploitation or a large age difference.
Q: What about contraception?
The new guidance says that before prescribing contraception, doctors and other health professionals should discuss several issues with the young person, including:
The emotional and physical implications of sexual activity, including pregnancy and sexually transmitted infections;
Whether the relation is mutually agreed as opposed to a situation of possible coercion and abuse;
The benefits of discussing the issue with a parent or carer.