Only some fertilized eggs are viable and implant in the uterus.
It takes about a week for this to happen.
Many fertilized eggs fail. Naturally. Only 1/2 of fertilized eggs naturally are viable to make it by nature through a full pregnancy and be born. The other 1/2 don't make it. Naturally, they don't survive, by the natural design of the human body. Which we know is God's design.
When does the spirit arrive to the dust body?
Well, at the end of our mortal life in these temporary bodies, the "spirit returns to God who gave it." --
"Then people go to their eternal home
and mourners go about the streets.
6 Remember him—before the silver cord is severed,
and the golden bowl is broken;
before the pitcher is shattered at the spring,
and the wheel broken at the well,
7 and the dust returns to the ground it came from,
and the spirit returns to God who gave it."
Ecclesiastes 12 NIV
God must give a spirit to the body, since the at the mortal death of the body, the spirit of the person returns to God, who gave it.
When does God put a spirit into the body? For those guessing it's at fertilizing of the egg, they are assuming God would put a spirit into both developing eggs that He knows will live, and also into flawed eggs He knows will soon die, naturally.
It makes more sense, and fits our natural human feeling from the body (designed by God), that only sometime after pregnancy actually begins in a way that humans are able to know, with a viable embryo implanted in the uterus and continuing successfully to live (viable, not doomed to die soon naturally) would a spirit be put in a body, unless you believe God would put a spirit into a body that He knows is only going to live a few days or few short weeks until naturally ending. One of the things our God-designed bodies tell us is that after a certain point in time, during pregnancy, it matters to most any woman if the baby is lost, after that time. Not before that certain time, but after a certain time. How long is that?
The Catholic idea that contraception (including a morning after pill type also then) is wrong to do seems extreme to most of us, but my friend explained it being that we are to trust God in this matter instead of trying to control it.
But that is inside of a marriage of course.
Consider a couple questions to think about other situations:
If you were a woman and some man tried to rape you, would it be wrong to resist?
Of course not.
Would it be wrong to convince the rapist to wear a condom?
Of course not.
But a condom is contraception.