The following is a discussion of some of the issues surrounding pregnancy, including sperm, fertilization, and sex determination of the fetus.
The fate of sperm
Sperm that has been ejaculated in to the vagina has the ability to swim by moving its long tail. With help from such things as muscle movement or the biochemical condition of fallopian tube secretions, sperm reaches the fallopian tubes from the vagina through the uterus. An egg, after being ovulated, can only live 18-24 hours, but sperm has a longer lifespan than eggs. It can live for 48-72 hours after ejaculation. So fertilization takes place when an egg is in the fallopian tubes and sperm arrives, or when an egg enters the fallopian tubes while there is live sperm.
The amount of semen that is ejaculated is approximately 3 mL and contains approximately 300 million sperm. Sperm do not like an acidic environment. The inside of the vagina is generally acidic, so sperm look for neutral or alkaline environments. The uterine entrance is neutral only 2-3 days before ovulation, or there is an increase in weak alkaline mucous. So a lot of sperm gather around the uterocervical canal, which is around the uterine entrance. Some sperm pass through cervical mucous and the uterine cavity and swim at a speed of 3 mm per minute toward the fallopian tubes. It takes 2-3 hours for sperm to reach the fallopian tubes. Only 150-200 sperm make it past the uterocervical canal and arrive at the fallopian tubes. Then, usually, only one sperm couples with the egg. In order for fertilization to take place, sperm have to be at the fallopian tubes before the egg. Couples trying to get pregnant should have intercourse a little before the estimated ovulation period.
When the sex is decided
Sex chromosomes determine sex. The two types of sex chromosomes are x and y. An egg only has x chromosomes, but sperm has both types. If the egg fertilizes with a sperm with x chromosomes, it is a girl, and if it fertilizes with a y chromosome, it is a boy.
Fission and implantation of the zygote
From the moment of fertilization, a zygote grows, and begins migrating toward the fallopian tubes. A zygote must pass through a fallopian tube's very narrow passageway. If the passageway is narrowed because of chronic inflammation, a large zygote is unable to pass. If implantation occurs inside the fallopian tubes, it becomes an extrauterine pregnancy. During this period, the zygote undergoes repeated fissions and arrives at the uterus 3-4 days after fertilization. At this time the zygote fissions into 12-58 cells. After fission occurs again, there is a split between the part that grows to be the fetus and the part that becomes the umbilical cord, amniotic fluid, amnionic membrane, and other things that help the fetus grow. Around this time, the cells begin implanting in the uterine wall. Pregnancy has occurred.