In law school we learned about the "fertile octogenarian" - a theoretical construct about what would happen to a property distribution scheme in an estate plan if you had some wacky birth order situation (ie: my great-uncle is 60 years younger than me). Back in 1995, this was largely theoretical. Not so today in age of reproductive medicine advances and frozen embryos. It is quite possible in 2012 to have a biological child of yours born 1, 2 or 5 or more years after your death. Did you mean to provide for this child that you never met in your Will? You Will likely says "after my spouse dies, everything to my issue". Your "issue" are your biological descendants, who this after-born child would be. While this may seem just weird, it is entirely possible in toady's age,
The US Supreme Court actually just considered this issue. A couple had frozen embryos, and about 9 months after the father died, the embryos were implanted and twins were eventually born. The mother applied for social security for the twins, and the issue came up of if the children are "children" for the purposes of social security. While it might seem very harsh to say that the children don't get benefits, when it comes to estate law you also need to bear in mind that an estate must end - it can't be held open forever waiting for heirs to come into being (an issue that didn't really exist 20 or more years ago).
In Astrue v. Capato the Supreme Court ruled that the Social Security Administration must look to state intestacy law to determine if a child would receive benefits under these circumstances.From a planning perspective, you should look to your own estate plan to determine who you think should be your descendants for purposes of distributing your estate, as merely relying on biology has a new meaning in 21st century medicine.