Re: whats this mean?
It means that technically the actual baby has only been inside of you for a month. When they count gestational age they count from the first day of your last period. That is usually two weeks before you ovulate and become pregnant. So technically during those two weeks you weren't pregnant, but that is where they decide to count from.
So if you actually conceived on the 15th of January (just pulling dates out of thin air ), then they would actually count from the first of January as that is probably the first day of your last period. So at that point, at the moment of conception, they would deem you two weeks along.