It depends upon where the egg is.
Ovulation may not have occurred yet,
making it a longer time for sperm to reach
the egg.
Here is an answer:
Question No. 3326 :
How long does it take for the human sperm
to reach the egg? How long can sperm and
egg remain viable for fertilisation?
Once sperm are deposited in the vagina,
they must travel through the cervical
canal, through the uterus, and up to the
egg in the upper third of the oviduct.
Spermatozoa can reach the distal uterine
(Fallopian) tube within 15 minutes after
semen is deposited in the vagina. The vast
majority of spermatozoa never make it.
Many are destroyed by vaginal acid or
drain out of the vagina. Others fail to
penetrate the mucus of the cervical canal,
and those that do are often destroyed by
leukocytes in the uterus. Half of the
remainder are likely to go up the wrong
uterine tube. Finally, only about 2000
spermatozoa reach the vicinity of the
egg.
Experiments have shown that freshly
ejaculated sperm are infertile; they must
be present in the female tract for at
least 7 hours before they can fertilise an
ovum. During the passage through the
female reproductive tract, the sperm
undergo a process of capacitation that
makes it possible to penetrate an egg. The
membrane of the spermatozoon head contains
a substantial amount of cholesterol, which
toughens it and prevents premature release
of acrosomal enzymes. Fluids of the female
reproductive tract slowly wash away
cholesterol and other inhibitory factors
in the semen. Most spermatozoa are fertile
for a maximum of 48 hours. An ovum can
stay in the uterine tube for at most 24
hours before it becomes incapable of
undergoing fertilisation.
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