I think that an objective line would be "the point at which, if the process is allowed to go on uninterrupted, it will be overwhelmingly likely to result in a baby." Gestation is a process, there are points in the process that occur all the time that don't result in a baby. Most fertilized eggs don't result in a baby for example. But there are points where it almost always does, and all of them fall after the fertilized egg attatches to the wall of the uterus. If you consider the process as one that exists to result in a baby, then the point in the process which, if left uninterrupted from that point on, it almost always will would be the point it becomes a human being. Just like a baby, if left to grow, will become an old person eventually, so we consider a baby as human as a working age adult. Because otherwise, we cant make either claim, a baby has no agency, it is not objective that it is human except by that same criteria.