I wonder if the 85% success rate statistic includes all pregnancies (including miscarriages or "spontaneous abortions’*) or just those that go to full term.
So if a man and woman- both fertile- have sex while she’s ovulating, why would she not get pregnant? And are some [obviously hetero and fertile] couples incompatible (i.e. either would have a much better chance of getting pregnant/impregnating if with another partner but their odds together reduced)?
*Spontaneous abortion may not be the correct term: I’m referring to very early miscarriages, as in those that abort within hours or a couple of days of conception and often the woman never even realizes she was pregnant. By some studies I’ve read these are actually the majority of pregnancies.
A few that come to mind: cervical stenosis (tight cervix), blocked fallopian tubes, poor intercourse timing, genetic abnormalities in the sperm or egg, lack of implantation due to issues with the uterine lining, hostile PH in the uterus/vagina/cervix, lack of or not enough cervical mucus, just missed the ball/wasn’t their month.
85% pregnancy rate refers to clinically detected pregnancy (i.e. the woman knows she is pregnant after peeing on a stick or a blood test), and includes the pregnancies that are terminated for whatever reason after the detection of that pregnancy (i.e. miscarriage, stillbirth and induced abortions).
A spontaneous abortion properly refers to any spontaneous loss of a pregnancy at any stage between implantation and birth.
A spontaneous miscarriage in the first or second trimester is usually referred to as a miscarriage, in the third trimester as a stillbirth.
Medically speaking, pregnancy starts at the implantation of the embryo (at this point a ball of cells) into the uterine lining, NOT at conception (when sperm meets egg). While you are correct in saying that many (?most) embryos that are created do not result in successful pregnancies, it is extremely difficult to know exactly what proportion of conceptions make it as far as implantation, and then again what proportion of successfully implanted embryos spontaneously abort before the woman misses a period or takes a pregnancy test.
We do know that between 15-25% of pregnancies that are confirmed by a pregnancy test are spontaneously aborted, and that only 20-30% of IVF treatments (where embryos are placed into the uterus) result in live births. However you look at it, it is a rather large rate of attrition.
To answer the second part of your post:
It is possible for both parties to have what is known as a balanced translocation of their chromosomes. This is where the person themselves has all the right genetic material, but sort of in the wrong order- like someone did a wonky cut and paste on their DNA. As this DNA is then split in half in order to make sperm or egg, it might be that the split leave sperm and eggs which don’t quite have the right stuff. Now put a “wrong” egg with a “wrong” sperm and you might not have the “right” total material to make a complete human. It might be that together there is simply no way for them to make a viable baby, while each, with a genetically "normal"partner, could have the potential of having a baby.
Or it could be something simpler- both parties have a recessive gene which is fatal in utero when the foetus inherits it from both parents. In this case they would have a 1/4 chance of a foetus which does not inherit the gene and is healthy, a 1/2 chance of a foetus which inherits one copy of the gene and is a carrier like mum and dad, and a 1/4 chance of having a foetus which inherits both copies of the gene and dies in utero. With a partner who does not carry the affected gene the odds shift to 1/2 unaffected, 1/2 carrier, no in utero deaths.
Missed the edit window, and this is important, in a hijacky, fighting ignorance, public information way. Sorry.
There is a delay between conception and implantation.
This is not a delay of seconds, minutes or hours, but a delay of days- between 6 and 8 days in most cases.
Identical twins, for example, form by a single embryo splitting into two during this period. It is possible, for example, to become pregnant with genetically identical twins even if only a single embryo is implanted during IVF.
Emergency contraception (Post-coital contraception, Plan B, the Morning After Pill, the Emergency IUD) works by preventing a fertilised embryo from implanting into the uterine lining, and thus preventing a pregnancy.
This is why hormonal emergency contraception (Levonelle, Plan B) can be effective in preventing pregnancy up to 72 hours after unprotected sex, and why a copper IUD can still be effective in preventing pregnancy if inserted 96 hours after unprotected sex.
This is also why some believe these methods of contraception to be abortifacients (although the medical profession disagrees, re-stating that in order to terminate a pregnancy, a pregnancy must first exist and that implantation is the starting point of a pregnancy).
This has been a message from irishgirl: reminding you of your contraceptive options!