It is important to introduce this subject with the position that you should always update your Will as your circumstances change. Finding out you or your partner is pregnant, having a new child/ adopting are all fundamental changes that should be carefully catered for in your Will.
Having said that, a number of potential scenarios can be catered for in your Will and as such it is possible to include provisions in your Will for unborn children as well as living family members, friends and charities, for example.
So, your Will can be used to accomplish many purposes and one of those may be to leave a gift to an unborn child. What is incredibly important here is to ensure that your wishes are clear. If you refer to your 'children' then this typically will include those children born to you/ living at the date of your death. This may or may not cover children that were born 'post' you making the Will, as what will be relevant then is whether you named your children specifically or referred to 'children' in more general terms.
If you also wish to include children with whom you/ you partner is pregnant with at the time of death, then, where you wish for those children to also be included, it is important to make that absolutely clear. There is very specific legal terminology that is used here in particular the French phrase of 'en ventre sa mere' literally translated as 'in its mother's womb' and this refers to the fetus in utero.
It is a commonly used expression in Legal English but does need to be specifically used for the position to be clear.
So, in summary, it is possible to include gifts in your Will to unborn children but the best practice advice is to in effect do this but treat your Will here as a holding Will until your child is born, as what you will then also need to do is create a new Will ensuring that you have also appointed a Guardian for that child, so you have someone in place to look after that child/ children should anything happen to you before they have attained the age of 18.