By Joanne Stone, Keith Eddleman, Mary Duenwald

If you suspect that you’re pregnant, you’re probably excited and want to know definitively whether you are pregnant. The first four weeks are important because the pregnancy is getting established as the implantation process is underway.

During these initial weeks, do the following:

  • Record your last menstrual period (LMP). Doing so helps your provider better estimate your due date (40 weeks from your LMP). Be sure to tell him if your cycles are irregular.

  • Begin taking a daily prenatal vitamin if you haven’t already. Check to see that the vitamin has at least 400 micrograms of folic acid to make sure your baby is as healthy as possible.

  • Check for ovulation. Some women know when they ovulate by a sensation known as mittelschmerz (German for “middle pain” due to the mild pain that may be felt when the egg is released from the ovary, typically 14 days after the LMP). Others know they’ve ovulated due to a change in the cervical mucus or a positive reading from an ovulation prediction kit.

    Fertilization usually occurs within the fallopian tube. The embryo starts as one cell. During the first week, that cell divides many times as it moves down the fallopian tube toward the uterine cavity.

  • Take your first pregnancy test. Pregnancy tests check for a hormone called human chorionic gonadotropin (hCG), which is produced by the embryo as it implants into the wall of the uterus — usually five to seven days after conception. By the time you miss a period, around ten days after conception, your pregnancy test will most likely be positive.

    Don’t be too concerned if you have a little spotting around the time when you would expect your period. This is most likely due to the embryo implanting in the womb (implantation bleeding).

At the end of the fourth week, your baby measures 0.2 inches (4 mm) in length.