Always measure correctly for your mattress to fit your cot or cot bed
It is very important that you have the correct size cot mattress to fit your cot or cot bed. the important rule is that the gap between the mattress and cot sides and or ends should never be more than 4 cm when the mattress is pushed tightly to the opposing side.
The danger is of entrapment if the gap is any any larger than this, for example your baby could easily get an arm or leg trapped in the gap which could be dangerous, and potentially fatal especially in very young babies.
Most cot and cot bed manufacturers will state the mattress size needed however if you are unsure or wish to check, Firstly assemble the cot before before measuring for the mattress. Where the cot is assembled measure the interior length & the width of the cot or cot bed base base. Ideally The mattress is to be a snug fit but you will need a little for sheets etc but make sure the gap is not smaller than 4cm as stated above .
Don’t have the mattress oversize either as this may impair the use of the drop side action on a cot if it has one and you still need to dress the mattress with bedding.
Also ensure that the gap between the cot mattress and the top of the cot is deep enough to stop your child from being able to climb out. It is recommended that cot mattresses depth measured at the side should not exceed 10 cm. on British cot mattresses.