Dr Laura Cassidy, an obstetrician and gynaecologist, explains what's normal and what's not for your monthly period.
Periods last around three to seven days. The average menstrual cycle lasts 28 days, although it's normal to have a cycle that's shorter or longer than this.
Women's periods can change. This doesn't necessarily mean there's a serious problem, but it does need to be checked.
"To get help, women should go where they feel most comfortable," says Dr Cassidy. "This could be a well woman clinic, a general practice or a community contraceptive clinic."
Missing a period could mean that you're pregnant. If you miss a period and you've had sex, take a pregnancy test to find out if you're pregnant.
"If you're not pregnant and you miss two or three periods, get medical advice," says Dr Cassidy. For many women, missed periods are due to upset hormones, and you may need help.
There can be other reasons for missed periods in some women, including:
"If you've been on the contraceptive pill for a long time, you might miss a period every now and again. It's not particularly significant," says Dr Cassidy. Some types of contraception, such as the contraceptive injection and the IUS (intrauterine system) called Mirena, can stop periods altogether.
It's not unusual for women approaching the menopause to miss periods, as ovulation becomes less regular. The average age for the menopause is 50 to 55, but sometimes women can have an early menopause in their teens, 20s or 30s (see Early menopause: real story).
Read more about the menopause.
Women whose periods stop before they're 45 or who are still bleeding when they're over 55 years old should get medical help.
Bleeding between periods or after sex
Bleeding between periods or after sex must be checked by a professional as it can be a sign of infection, an abnormal cervix or, in rare cases, cancer.
"If you've got bleeding between periods or bleeding after sex, there might be some abnormality of the cervix," says Dr Cassidy.
The abnormalities can be harmless, for example polyps, or they can be due to an infection such as chlamydia.
Find out what causes bleeding between periods.
If you're worried, your community contraceptive clinic, sexual health clinic or genitourinary (GUM) clinic can offer advice, testing and treatment. Find your local sexual health clinics.
Sometimes, taking a low-dose contraceptive pill can cause bleeding between periods. This can be corrected by changing pills. Find out about different methods of contraception.
The National Cervical Screening Programme is a scheme to detect abnormalities in the cervix that could lead to cancer if untreated. Women aged 25 to 49 are offered screening every three years. Women aged 50 to 64 are offered it every five years.
Sudden period changes
If your periods suddenly change - for example, they become heavier or longer - get advice. This is especially important for women over 40.
"For most women under 40 with slightly irregular cycles, these symptoms do not have a serious cause," says Dr Cassidy.
"But in women over 40, if periods become much heavier, or are lasting longer, or if they're having erratic bleeding, this needs to be investigated."
Changes in women over 40 can be associated with endometrial cancer, cervical or endometrial polyps, or a pre-cancerous condition (hyperplasia), which can be treated if caught early.
Bleeding after the menopause
If you have any bleeding after menopause, seek medical attention urgently. "After a woman has had a year with no periods, she should not bleed," says Dr Cassidy. "If you do, seek advice right away. Don't wait to see if it happens again."
Read one woman's experience of bleeding after the menopause.
Some women taking HRT to control the symptoms of menopause will have bleeding or spotting. This depends on the type of HRT you are taking. If you take the cyclical, or sequential, preparation then you'll have a regular period-type bleed, known as a withdrawal bleed. See a doctor if you notice any bleeding between these withdrawal bleeds, whether it happens after sex or at any other time.
If you take the continuous combined preparation, you won't have a regular bleed but you may notice spotting. Dr Peter Bowen-Simpkins, consultant gynaecologist and medical expert for the charity Wellbeing of Women, says: "Women taking continuous combined HRT are period-free, but up to 50% of women may have slight spotting in the first year of taking it. If this spotting is persistent or heavier than just spotting, it should be reported to your doctor immediately."
If you switch from cyclical to continuous combined HRT, and you notice bleeding, you should stop. "In these circumstances, you will need to return to the sequential preparation and report any untoward bleeding, should it occur," says Dr Bowen-Simpkins.
Find out about self-help methods to help you cope with menopause.
Discharge during your period
It's normal to have vaginal discharge (secretions), and it's normal for the discharge to change during the menstrual cycle.
"A clear or creamy white discharge is normal, and often makes slightly yellowish stains on the underwear," says Dr Cassidy. Around ovulation, the discharge becomes stretchy, a bit like raw egg white.
See a doctor if you have discharge that's green, blood-stained or smells, as this could be a sign of infection.