There is definitely enough. Most of the sperm in fact comes from the pre-cum.
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Whoever made the previous answer, has no clue what he is talking about.
Sperm and ejaculatory fluid are made at entirely
different places in male body.
"Sperm" is a term describing sperm cells, while pre-ejaculatory fluid is body's natural way of removing acidity from urethra (that is present because of urine -it pases trough the same canal as sperm).
Pre-ejaculatory fluid or "precum" is made in
Cowper's Gland, and serves to protect sperm that is on its way to leave the body of the male. It also serves as a natural lubricant.
Also, pre-ejaculatory fluid contains
NO sperm,
UNLESS it "picks it up" from the previous ejaculation. And in those cases that there is some sperm left, it is accepted that it is around the 2% of total ejaculation. (Full healthy ejaculation contains around 200 millions of sperm cells)
Urination before sex kills leftover sperm in your urethra and washes out anything else.
It is still debated whether amount of sperm that remains in pre-ejaculatory fluid after ejaculation is enough for pregnancy, but common sense and research thus far claim that chances for it are
extremely small, because of the sheer numbers of sperm cells it takes for a successful pregnancy, and because pre-ejaculatory fluid alone is not a good composition for sperm to live in.
Full ejaculation contains liquids that "feed" the sperm, which is absent in pre-ejaculation fluid alone.
And to those claiming that "it takes only one sperm to fertilize and egg", I must
disagree.
It is like saying that it took 1 allied soldier to kill Hitler in World War 2.
But would it have been enough if Allies sent only 1 soldier.... or 1 battalion on Normandy, France, Germany and the Berlin?
Would such a small force be enough to break trough and kill Hitler?
Likely not.
Sperm is simple and stupid.
It's main advantage is in
numbers. Without them, pregnancy has a minimum chance to occur.
It is medicaly accepted that men with less than 15 million sperm/ml are
infertile.