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    A single-cell zygote forms, divides, and doubles every 12 hours.

    Close up of a zygote in the third week

    Your baby’s development

    Conception, from A to Zygote.
    By this time, you might have conceived already. Fertilization occurs when an egg and sperm connect and form a single cell, called a zygote. During your 3rd week of pregnancy:

    • The microscopic, one-celled zygote repeatedly divides and grows at an astounding pace.
    • This process continues as the total number of cells in your baby doubles every 12 hours. While this division occurs, your zygote is moving down the fallopian tube to your uterus.
    • Your baby already has all of the genetic material she needs for life — half of the 23 pairs of chromosomes from you and half from your partner.
    • Your baby's gender is already determined.
    A glass of orange juice next to an orange cut in two

    Your nutrition and health

    Refocus your nutrition for pregnancy.
    Calories provide energy to help your baby grow.

    • Folic acid is especially critical during the first 8 weeks of pregnancy for central nervous system and fetal brain development, but remains important throughout the course of your entire pregnancy.

    During this formative stage:

    • Continue to take your doctor-recommended folic acid supplement or prenatal multivitamin.
    • Drink plenty of fluids.
    • Respond right away if your body indicates that you should slow down or stop.
    An adult swimming in the pool

    Things to think about now

    Exercise, exercise, exercise!
    If you're already in good condition and regularly exercising, pregnancy shouldn’t prohibit you from continuing a safe exercise routine. But if you’re just beginning, always ask your doctor before starting any new physical exercise.

    Good exercises for women in their 1st trimester of pregnancy include:

    • Walking
    • Swimming
    • Aerobic Activity

    The American Congress of Obstetricians and Gynecologists (ACOG) has some good recommendations here.

    What happens next week

    You actually find out you’re pregnant.
    You'll likely miss your period and find out you're pregnant. The zygote embeds, the placenta thickens, and your baby continues to grow.