Why is it so hard for women to conceive, biologically speaking?

I don't know where the 25% statistic comes from - I've heard it before but I have no idea what that's based on. It seems a bit bogus to me. I've certainly never had any trouble getting pregnant. My first one was unplanned, when we took one little chance. The second one took 3 months of trying, but only because we were traveling or sick around the time of ovulation in the first two months. Once we got the timing right, I was pregnant. This time I got pregnant on our first try. Obviously that 25% doesn't apply to me!

Even if that number is accurate, there a LOT of different factors involved here. One is that humans (compared to other species) wait until much later in life to try to conceive. A woman's natural fertility decreases as she gets older, so the longer she waits the more likely she is to have trouble conceiving. This is true for men as well, since sperm count and sperm virility may decrease as the man gets older.

Many women do things before ttc that interfere with their chances. Spending years on hormonal birth control, catching STDs, etc. can cause problems for some women. Smoking, being overweight or underweight, using illegal drugs, and lots of stress can also make it harder to conceive. Past abortions can make it harder to conceive when she finally decides she wants a child. Other species obviously don't have these issues.

As for why conception doesn't occur every time - it could be any of the things you said, and again there are many possible causes. Some women have vaginal mucus that is not as receptive to sperm as it ought to be (due to hormonal imbalance, bad genes, medical problems in the past, etc.), causing the sperm to not live as long and therefore have trouble reaching the egg or penetrating the egg. Some fertilized eggs fail to implant (I would consider this an early miscarriage, not failed conception, although the woman would probably never know she was pregnant), due to a uterine lining that isn't healthy/receptive or due to genetic problems with the newly conceived baby or just because it doesn't get there fast enough before the hormones switch over and trigger menstruation.

Major deciding factor in pregnancy - I don't think there is one. There are too many factors and they all have to work together. Having more sperm increases the chances, but one very strong and lucky sperm can do the job too. The woman's body also must be receptive to the sperm (fertile mucus, healthy uterine lining, etc.) and the egg must be healthy and in the right place at the right time. If any of those factors is lacking, the chance of pregnancy goes down.

Animals don't necessarily get pregnant right away, that's why they mate several times, in case one time failed. Even then, not all the time does it work, some animals just aren't compatible biologically and can't produce offspring, even when there is nothing wrong with either partner. I've seen people try to mate horses or dogs and nothing happens, but both partner go on to mate with new partners and have babies. Science can't yet fully explain why this is, they just suspect it has something to do with the male's genes and female's genes don't create a healthy genetic makeup, therefore, no pregnancy.

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