A child car seat is one of the most important purchases you'll make for your baby to keep them safe, so it's crucial that you get the right one. So keep watching for everything you need to know before heading out to the shops. Children's bodies are different to adults and far less force is needed to cause serious injury. That's why getting the right child car seat to fit their growing body as it changes is so important. Child car seats are split into groups from 0 to 3, with each group designed to provide the right protection as your child grows. It's so important that the law now says that you must use a seat for your child until they're 12 years old or 135 centimetres tall. We'd go even further and recommend your child stays in a seat until they're 150 centimetres tall. Before your baby's born you'll need a rearward facing group 0 plus child car seat and remember you won't be able to leave hospital without one. This kind of seat provides the best protection for babies and toddlers. We recommend you keep your child rearward facing until they're at least 15 months old. Reach the weight of 13kg or the crown of their head is level with the top of the car seat. Baby's heads are about a third of their body weight and their neck muscles are weak. So if they're forward facing there's risk of their head being pulled forward by the force of a crash causing severe neck and spine injuries. Once they've grown too big they'll need a group 1 child car seat which is 4, 9kg to 18 and that's around 9 months old to 4 years. Once your child has outgrown a group 1 seat they'll need a group 2 or 2, 3. This is for children from 15kg which is around 3 years old. The recommended weight for each group overlaps so hold off until your child reaches the top of the range before swapping. For a group 1, 2 or 3 child car seat your child will also have outgrown it if his or her eyes are level with the top of the seat. Once your child's old enough for a group 2, 3 seat it's always good to get a high back to one rather than a backless one. Our tests have proven that inner crash a seat without side protection will leave your child's head, neck and body at risk of hitting the side of the car causing serious injury. You can get child car seats that combine more than one group such as a 1, 2, 3 but our tests have found that not all child car seats protect your child at every single stage. I mean there's a big difference between a 9kg baby and a 36kg child. If you really want to use this kind of seat then make sure you take a look at our Best Buy and Don't Buy Car Seats. As well as weight there are three important things you need to consider when buying a child car seat. Firstly what car will you be using it in? Not all child car seats fit in all cars so it's vital you take along the car you're going to be using so you can check it fits properly. You should also make sure it will fit in any other cars you plan to use it in such as grandparents or childminders. Some manufacturers websites will list what cars their seats are expected to fit in so it's worth checking before you head to the shops. Check whether your car has icifix mountings. icifix is a standard system of fitting which is in most new family cars and it basically means you can fit the child car seat without the use of a seat belt. icifix seats have two or three points that can be fixed. There are two at the base and a third either as a strap to go over the back of the car seat or a drop down foot. We recommend looking for one with three points as it's generally more secure if fitted correctly. If you get one with a drop down foot just check that you haven't got underfloor storage as this will stop the brace from working. Click the link below where you can look at our fitting videos where we explain this further. Will you be transporting more than one child? If you already have child car seats make sure you take them along to the fitting so that you can check the new one will fit alongside them. Or if you can't take them with you at very least mention it to the retailer. How tall are you? Tall people in the front seats can affect how much room there is for real-ward facing child car seat so keep this in mind when shopping. Never buy a child car seat second hand or buy unofficial replacement parts. It may have been weakened by an accident or have wear and tear that could make it less safe for your child. Even if you're using one from a friend or family member it may have damage that you can't see. We've tested hundreds of child car seats and found big differences between a good one and a bad one. Take a look at our video guide to what makes a best buy. You can find all our information on child car seats and best buys that have been able to withstand our tough lab tests that are higher than the UK standard by visiting witch.co.uk Click on the links below for more on choosing a child car seat, how we test child car seats and our car seat reviews.