The research behind this is wild. Your sperm carries a set of instructions that tell your genes when to turn on and off. A Duke University study found that THC rewrites those instructions. The more weed in your system, the bigger the changes. It goes straight for the genes your future embryo needs in its first week of life. I had to read the "day 3 crash" part twice. For the first three days after fertilization, an embryo runs entirely on the mother's DNA. Day 3, the father's genes switch on. If those genes carry cannabis damage, the embryo just stops growing. Fertility doctors see this happen in their labs: embryos that fertilized fine and looked healthy on day 2 go completely still by day 5. Boston University tracked 1,535 couples trying to have a baby. Men who smoked weed once a week or more doubled their partner's miscarriage risk. That number held up even when the woman herself never touched cannabis. And the miscarriages clustered in the first 8 weeks, right when the father's damaged DNA would be doing the most harm. Duke also found that the specific genes THC alters in sperm overlap with genes linked to autism. One of those genes, called DLGAP2, helps brain cells communicate with each other. It was changed in cannabis users' sperm. When researchers bred THC-exposed male rats and checked their offspring, the same altered gene pattern showed up in the pups' brains. The damage crossed a generation. Weed has gotten way stronger over the last 30 years. THC content was about 4% in the 1990s but nearly quadrupled to 15% by 2018, and modern dispensary strains regularly sit at 20-30%. Concentrates go up to 95%. Quitting for about 11 weeks (one full cycle of sperm production) reverses some of the DNA changes. Not all of them. Duke's lead researcher says men should stop at least 6 months before trying for a baby. Half of your kid's genetic blueprint comes from you, and right now, THC is editing that blueprint before conception even happens.