In a very fertile woman, one drop of sperm can do the trick. However getting pregnant can be easy for some, while it can be impossible for others.
It depends a lot if the woman is ovulating, those are her most fertile days. If someone is having difficulty getting pregnant, it is advised to make love every night for about one week.
Which are the woman's most fertile days?
A woman can be most fertile 6-14 days after the first day of her period.
How does she know she's close to ovulating?
She will know because her vaginal discharge will be more watery and creamy. The ovulation discharge can look like egg whites, when she wipes herself in the bathroom. That is a sign that ovulation should begin in the next 48 hours.
Once an egg is released it can live for 12 hours, meanwhile it waits to be fertilized and if no sperm is available the egg will die and move to the uterus to be released in the next menstrual period.
What happens if a woman ovulates 24 hours after sex?
It is likely she will become pregnant because sperm stays in the woman's reproductive track, for at least 3 days. It has been scientifically found that the "female" sperm or basically sperm labeled to be born into female, can live up to 5 days in a woman's reproductive track.