First, how likely is it for sperm to connect with egg to make a zygote?
"The probability of conception ranged from 10% when intercourse occurred five days before ovulation to 33% when it occurred on the day of ovulation itself." (1)
"Researchers found that couples who had sex every day had a 25% chance of conceiving, while couples who had sex every other day had a 22% chance. " (9)
"Conception is most likely to occur in the first month of trying (about a 30% conception rate)." (10)
Let's be generous, and go with the 30%. That means, if 100 women have sex for a month, 30 of those women will have a fertilized egg.
Next- will that zygote implant?
"John Opitz, a professor of pediatrics, human genetics, and obstetrics and gynecology at the University of Utah, testified before the President's Council on Bioethics that between 60 and 80 percent of all naturally conceived embryos are simply flushed out in women's normal menstrual flows unnoticed." (2)
"Between 20 per cent and 80 per cent of newly fertilised eggs (zygotes) fail to implant in the uterine wall." (3)
20-80 % is a wide wide variety. But it's hard for us to know, because it's so difficult to know if a woman has conceived before the pre-embryo implants. To make it easier, I'll just go with 50%.
So of the 30 women who have fertilized eggs, only half of them will have those eggs implant. That leaves us with 15 women.
Next question: Will the pregnancy make it through the first trimester/ to 20 weeks?
This can be a difficult question. The longer the pregnancy lasts, the less likely a miscarriage is to happen. For instance, this website (http://www.pregnancyloss.i
"The overall risk of miscarriage once a woman knows she is pregnant is 12 to 15 percent, the researchers note in the journal Obstetrics and Gynecology."
"For women in childbearing years, the chances of having a miscarriage can range from 10-25%, and in most healthy women the average is about a 15-20% chance." (5)
Since the two links both say 15%, I'll stick with that number. So we have 15 pregnant women and 15% miscarry, which leaves us with 12.75... 13 women who are still pregnant, since it's hard to have 3/4th of a woman!
After 20 weeks, a pregnancy loss is called a stillborn. How many times does having a stillborn happen?
"A baby is stillborn in about 1 in 200 pregnancies." (6)
"Each year in the United States about 25,000 babies, or 68 babies every day, are born still. This is about 1 stillbirth in every 115 births." (7)
Even if we average these two, it leaves us with 0.63% of pregnancies ending in stillbirth.
"Stillbirth rate is 1%." (8)
This rate is not high enough to affect our small sample of pregnant women. If we had a larger group, it would, but for now it wont work.
So in the end, we've got 100 women having sex for a month, and only 30 women will get pregnant, and only 12 women will carry pregnancies to term. This of course, does not factor in things like contraceptive (which would make our number of conceptions much much lower), extreme / elevated stress, which could lead to more complications, or a lack of health care.
If they are having sex everyday, that means we have up to 3,100 cases of sexual intercourse which result in 12 babies. I don't know how anyone can claim making a baby is the main purpose of sex when it happens so infrequently!