In week 4 of pregnancy, your body is beginning to form the placenta and amniotic sac.
Symptoms like abdominal pressure and tender breasts may appear this week, and as the cluster of cells that will soon become your baby burrows into your uterine lining, you may also spot some implantation bleeding. (But if you don't notice any symptoms at all yet, that's completely normal too.)
Your Baby at Week 4
It’s implantation time! Now no bigger than a poppy seed (though a whole lot sweeter), the ball of cells that will become your baby has completed its weeklong voyage down your fallopian tube and has claimed its berth in your uterus by tunneling into the sticky uterine lining. Nestled in your uterine home, this miniature miracle has established the cozy connection that will bind the two of you for the next eight plus months and forever after.
But that’s just the beginning of the action this week. In the works is the amniotic sac that your baby will call home sweet home. And at last! … the sign you've been waiting for: your little embryo starts to produce HCG — the just-for-pregnancy hormone that’ll turn your pregnancy pee test positively positive. You’re officially expecting!
Though your embryo is only a tiny dot, it has already started to differentiate into three layers, each with its own job assignment. The inner layer, called the endoderm, will become the digestive system, the liver, pancreas, bladder, and thyroid gland.
The middle layer — the mesoderm — will grow into the heart, bones, kidneys, muscles, and blood vessels. The outer layer, called the ectoderm, will develop into the brain, nervous system, skin, hair, and eyes. It’s hard to believe, but by the end of your fourth week of pregnancy, all the building blocks for your baby’s vital organs are firmly in place.
At a Glance
4 weeks pregnant is how many months?
If you’re 4 weeks pregnant, you're in month 1 of your pregnancy. Only 8 months to go! Still have questions? Here's some more information on how weeks, months and trimesters are broken down in pregnancy.
How big is my baby at 4 weeks?
Despite its extremely tiny size — no longer than 1 millimeter and no bigger than a poppy seed (think about that as you eat your morning bagel) — your little embryo is busy setting up house.
Placenta and embryo begin forming
While you may have just started to wonder whether you're pregnant, your soon-to-be baby has already found its home: The blastocyst has completed its journey from your fallopian tube to your uterus.
Once there, it burrows into your uterine lining and implants — making that unbreakable connection to you that'll last the next eight months (and a lifetime after that).
As soon as that little ball of cells is settled in your uterus, it will undergo the great divide, splitting into two groups. Half of what's now called the embryo will become your son or daughter, while the other half forms the placenta, your baby's lifeline — which channels nutrients and carries waste away until delivery.
Development of the embryo and amniotic sac
While the amniotic sac (also called the bag of waters) forms around it, so does the yolk sac, which will later be incorporated into your baby's developing digestive tract.
The embryo now has three distinct layers of cells that will grow into specialized parts of your baby's body. The inner layer, known as the endoderm, will develop into your baby's digestive system, liver and lungs. The middle layer, called the mesoderm, will soon be your baby's heart, sex organs, bones, kidneys and muscles. And the outer layer, or ectoderm, will eventually form your baby's nervous system, hair, eyes and outer layer of skin.
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