Your Baby's Development
This may sound strange, but you're still not pregnant! Fertilization of your egg by the sperm will only take place near the end of this week. (Read more about fertilization in the Your Body section below.)
A baby's gender is determined at the moment of fertilization. Out of the 46 chromosomes that make up a baby's genetic material, only 2 — 1 from the sperm and 1 from the egg — determine the baby's sex. These are known as the sex chromosomes. Every egg has an X sex chromosome; a sperm can have either an X or a Y sex chromosome. If the sperm that fertilizes an egg has an X chromosome, the baby is female; if it has a Y chromosome, the baby will be a boy.
Your uterine lining, which will nourish the baby, is developing, and your body secretes follicle-stimulating hormone (FSH), which stimulates an egg to mature. At the end of this week, you will be at the midpoint of your menstrual cycle (if you have a regular 28-day cycle), and ovulation happens (the ovary releases an egg into the fallopian tube).
This is when you're most likely to conceive. If you have sex without using birth control around the time that you ovulate, you can become pregnant. After the male ejaculates, millions of sperm travel through the vagina, and hundreds make it to the fallopian tube, where the egg is waiting. One sperm generally succeeds in penetrating the egg, and fertilization takes place. When that happens, you will be pregnant — but you won't feel any body changes just yet.