1. Pregnancy and chance

Pregnancy is always a matter of chance. A normal couple trying to conceive in their 20s has about a 25% chance of achieving pregnancy in any one month (for couples in their 30s it's about 15% per month). These figures are averages: many normal couples have a lower chance and many others a higher chance.

Sooner or later a couple will achieve pregnancy if: 1) they have a reasonably high monthly-chance-of fertility (say, more than 5% per month), and 2) a reasonable period of trying (2 years). If pregnancy has not occurred within a year or two of trying then it's more likely that the monthly chance of pregnancy will be less than 5%. Tests should then be done.

For normal fertility, sperm deposited in the vagina must (move) through the cervix, uterus and fallopian tubes to meet an egg that has been carried from the surface of the ovary to the mid-part of the fallopian tube. The resulting embryo (strictly speaking it's still a "pre-embryo") develops for the first five days in the fallopian tubes then travels to the uterus, where it floats and develops for 24-48 hours, and then it implants in the lining of the uterus - thus establishing pregnancy. A few days before the period is missed, a blood pregnancy test will be positive.

It's about at this time of implantation, a week after fertilization, that the first few cells in the center of the "embryo" actually differentiate into what will be the fetus; all the other cell will form the afterbirth, or placenta.