Pregnancy Week by Week – Weeks 1 to 4

You’ll want to take care of your health over these next few months because you’re caring for both yourself and your developing baby. Here's what you should know.

Congratulations! You’re embarking on an exciting new adventure. Over the next 40 or so weeks, you’ll watch your body change and grow as it prepares to nourish and house your baby. A lot of anticipation, excitement, and planning go into becoming a parent. You’ll want to take special care of your health over these next few months because you’re caring for both yourself and your developing baby now.

Here’s a peek inside the womb during those very early weeks of pregnancy.

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Weeks 1 - 2

Pregnancy starts when an egg travels down the fallopian tube during ovulation and meets up with a sperm. Your doctor will track your pregnancy, and determine your due date, from the first day of your last menstrual period, which starts about 14 days before you ovulate. That means you’re not actually pregnant at the start of your pregnancy. The egg won’t be fertilized until around week two.

Your body

Nothing is happening in your body just yet. You’ll have to wait a few more weeks before you start noticing the early signs of pregnancy.

Weeks 3 - 4

The cells that will become your baby are dividing rapidly now. During week four, a ball of about 100 cells will burrow into the lining of your uterus in a process called implantation. The cells will form into layers, which will eventually make up all your baby’s organs and tissues. The inner layer will grow into the lungs, stomach, intestines, and bladder. The middle layer will form the heart, blood vessels, bones, and muscles. The outer layer will become the baby’s brain and nervous system, along with the hair and skin.

A sac will form around the embryo and fill with fluid. This is called the amniotic sac. Its job is to protect your growing baby. A flat organ called the placenta will also develop to nourish your baby and remove wastes from his or her body.

Your body

Once the embryo implants in your uterus, it produces a hormone called human chorionic gonadotropin (hCG). The pregnancy tests you take at home or in your doctor’s office detect this hormone and confirm your pregnancy. hCG also signals your ovaries to stop releasing an egg each month, so your period is likely to stop at this point. You might also see some spots of blood on your underwear. Spotting is a sign that the embryo has implanted in your uterus.

Your baby

The embryo is still tiny. By week four, it will measure only ¼ inch long. As tiny as your baby is, his or her gender has already been determined by sex chromosomes. A baby with two X chromosomes is a girl. One with an X and a Y chromosome will be a boy.

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What to remember

If you haven’t already started taking a prenatal vitamin containing folic acid, get on one now. Taking 400 micrograms (mcg) of folic acid each day can reduce your baby’s risk of birth defects like spina bifida and anencephaly. Also prep your body by embarking on a healthy diet and exercise plan. You want to be in your best possible shape when you conceive.

Stop any habits that could be bad for your baby — like smoking and drinking alcohol. Smoking is especially harmful to both you and your baby during pregnancy. Cigarettes can cause premature birth, low birth weight, birth defects like a cleft palate, and even miscarriage or stillbirth. It’s a habit you’ll want to stub out as long before you conceive as possible.

Now is also the time (if you haven’t done so already) to schedule a preconception visit with your ob/gyn. Find out whether medicines you take are safe to continue during pregnancy, and talk with your doctor about any genetic risks that might affect your newborn’s health.

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