You should always update your Will as your circumstances change. Finding out you or your partner is pregnant, having a new child or adopting are all fundamental changes that should be carefully catered for in your Will.
It is possible to provide for unborn children in your Will, and the important thing is to ensure that your wishes are clear. If you refer to your 'children', this may or may not cover children that were born after you made the Will.
If you wish to include a child that you/your partner is pregnant with at the time of death, very specific legal terminology needs to be used for the position to be clear. The French phrase 'en ventre sa mere', which is literally translated as 'in its mother's womb', is required to ensure unborn children are included in your Will.
Best practice advice is to treat the Will as a holding Will until your child is born. You then need to do is create a new Will and appointed a Guardian for that child, so you have someone in place to look after them should anything happen to you before they reach the age of 18.