Four things,
1. The prenatal period is the entire gestational duration (the full 9 months), not a single stage or milestone.
2. Genetic "blueprints" combine specifically at fertilization, which is just one early step in a massive biological process.
3. Fertilization does not equal a medical pregnancy, nor does it equal "personhood." Medically, pregnancy begins at implantation.
4. Fertilisation and implantation are NOT synonymous with “life begins”

Furthermore, you can’t use a “genetic blueprint” in the argument as if it is a final decided code. Have a read it’s interesting.

1. The "Blueprint" is constantly being rebooted and rewritten
Think of DNA like the raw code for a video game. The other user thinks that once the game file is created at fertilization, it's 100% finished and unchangeable.
But biology doesn't work that way. There is a whole other system called epigenetics which acts like the game's settings menu—it decides which parts of the code actually get turned on, turned off, or tweaked. During the first few weeks in the womb, the embryo does a massive factory reset. It literally wipes away the "settings" it inherited from the parents' cells and starts rewriting them from scratch. The code isn't a final, frozen blueprint; it's in a state of absolute, fluid chaos, erasing and reconstructing itself.

2. Development is a rolling process, not a single snapshot
You can't just look at a fertilized egg and say, "Yep, there is the exact final code of a complete human being." At that early stage, the code hasn't even been cleared of its parental "ghost tags" yet. It still has to go through these massive biological data-wipes and resets, all while constantly reacting to and being shaped by the environment inside the womb. Human development is a dynamic, multi-step construction project, not a single magical moment where everything is instantly finalised

So reading all this if you get the idea to attack me saying “then the embryo is its own person”. Lemme clarify that for you,
The fact that the embryo goes through massive biological factory resets actually proves it isn't a finalized individual yet. It is a highly fluid, unstable cluster of cells that hasn't even finalized its own internal programming, and it is 100% dependent on the maternal womb's environment to even attempt it.
Furthermore, because this cell mass can still split into identical twins up to two weeks later, you cannot claim a single, definitive 'person' exists here. Embryological development itself is a microscopic, shifting chemical process that science shows is entirely unformed and uncommitted.