Experts are still unsure exactly how much - if any - alcohol is completely safe for you to have while you're pregnant, so the safest approach is not to drink at all while you're expecting.
The Chief Medical Officers for the UK recommend that if you're pregnant or planning to become pregnant, the safest approach is not to drink alcohol at all to keep risks to your baby to a minimum.
Drinking in pregnancy can lead to long-term harm to the baby, with the more you drink, the greater the risk.
When you drink, alcohol passes from your blood through the placenta and to your baby.
A baby's liver is one of the last organs to develop and doesn't mature until the later stages of pregnancy.
Your baby cannot process alcohol as well as you can, and too much exposure to alcohol can seriously affect their development.
Drinking alcohol, especially in the first three months of pregnancy, increases the risk of miscarriage, premature birth and your baby having a low birth weight.
Drinking after the first three months of your pregnancy could affect your baby after they're born.
The risks are greater the more you drink. The effects include learning difficulties and behavioural problems.
Drinking heavily throughout pregnancy can cause your baby to develop a serious condition called foetal alcohol syndrome (FAS).
Children with FAS have:
Drinking less heavily, and even drinking heavily on single occasions, may be associated with lesser forms of FAS. The risk is likely to be greater the more you drink.
It may not be as difficult as you think to avoid alcohol completely for nine months, as many women go off the taste of alcohol early in pregnancy.
Most women do give up alcohol once they know they're pregnant or when they're planning to become pregnant.
Women who find out they're pregnant after already having drunk in early pregnancy should avoid further drinking.
However, they should not worry unnecessarily, as the risks of their baby being affected are likely to be low.
If you're concerned, talk to your midwife or GP
If you do decide to drink when you're pregnant, it's important to know how many units you are consuming.
One UK unit 10 millilitres (ml) - or eight grams - of pure alcohol. This is equal to:
You can find out how many units there are in different types and brands of drinks with the Drinkaware unit and calorie calculator.
If you have an Android smartphone, iPhone, iPad or iPod touch, you can download the free One You Drinks Tracker from Google Play or the iTunes App Store. It allows you to keep a drinks diary and get feedback on your drinking.
Click here to read more about alcohol units.
If you have difficulty cutting down what you drink, talk to your midwife, GP or pharmacist.
Confidential help and support is also available from local counselling services:
Read more advice on cutting down your drinking here.
Find maternity services near you.
Using illegal or street drugs during pregnancy, including cannabis, ecstasy, cocaine and heroin, can have potentially serious effects on your unborn baby.
If you regularly use drugs, it's important to tackle this now you're pregnant. It's best not to stop abruptly without first seeking medical advice as there may be withdrawal problems or other side effects.
If you took a drug without realising you were pregnant on a one-off occasion, try not to worry - it's very unlikely to have affected your baby.
However, if illegal drugs are part of your life, getting help can really improve the outlook for you and your baby.
If you use drugs, it's important to seek help straight away so you can get the right advice and support.
You can get help from:
They won't judge you and can help you access a wide range of other services, such as antenatal and family support.
You can also contact FRANK for friendly, confidential drugs advice, including information on the different types of help available. The FRANK helpline is open every day, 24 hours a day on 0300 123 6600.
For more information about what drug treatment entails, see Public Health England.