That same argument applies to all the babies who die between being born and turning 1. It's more an argument against God than an argument against rights starting from the first moment of life. If we assume God doesn't exist, then the argument doesn't really work.
If we assume God does exist, one possible rebuttal is that the purpose of life isn't to test moral virtue. For example the purpose of life could be to provide an opportunity to know, love, and glorify God. But it's just an opportunity, not the only opportunity. After death there continue to be opportunities for that, so people who die immediately after ensoulment still have an opportunity for that.
That same argument applies to all the babies who die between being born and turning 1. It's more an argument against God than an argument against rights starting from the first moment of life. If we assume God doesn't exist, then the argument doesn't really work.
If we assume God does exist, one possible rebuttal is that the purpose of life isn't to test moral virtue. For example the purpose of life could be to provide an opportunity to know, love, and glorify God. But it's just an opportunity, not the only opportunity. After death there continue to be opportunities for that, so people who die immediately after ensoulment still have an opportunity for that.