illustration of developing fertilized egg with three germ layers

4 weeks pregnant

For information about weeks 1 through 11 of a twin or multiple pregnancy, tap here.

While you haven’t been pregnant for very long, there are some big changes happening for both you and your baby in week four! 

How’s Baby?

Baby is now as big as a poppyseed, and has entered the embryonic stage of pregnancy! Starting around this time, the neural tube, which develops into the brain and spinal cord, is forming. Baby is also made up of three distinct layers right now: the endoderm, which will becomes their lungs and most of the vital organs; the mesoderm, which will grow into the heart and skeleton; and the ectoderm, which will make up the nervous system.

And not only are you growing a tiny human, you’re also growing an organ that will help nourish your little one throughout the next several months! Your placenta is starting to form, spreading its roots into your uterine wall, before it begins its task of transporting nutrients to the baby and carrying away waste. For now, while the placenta settles in and develops, Baby is being nourished by the yolk sac from your egg, and the placenta is busy reaching out and making a connection with your blood supply.

What's new with you?

By the fourth week of pregnancy, many people start to have an inkling that they’re pregnant, whether due to bloating caused by the hormone progesterone, mild discomfort from the fertilized egg implanting in your uterus, or other side effects that are starting to pop up. You could also start to be experiencing mood swings. This may not be fun, but it is normal. This increase in estrogen and progesterone also reduces the speed with which your digestive system works in an effort to maximize nutrient absorption and most effectively supply and nurture your growing baby. You may experience aversions to certain foods as a result, but don’t worry, this is also perfectly normal. And have you started taking a prenatal vitamin yet? If not, this is the time to start.

Reviewed by the Ovia Health Clinical Team
Read more
  • Sir John Dewhurst. Dewhurst's Textbook of Obstetrics and Gynaecology. 8th ed. Keith Edmonds. John Wiley and Sons Ltd, 2012. Print.
  • G V Pepper, S C Roberts. "Rates of nausea and vomiting in pregnancy and dietary characteristics across populations." Proceedings of the Royal Society B: Biological Sciences. 273(1601): 2675-2679. October 22, 2006. Web. 
  • Mayo Clinic Staff. "First trimester pregnancy: what to expect." Mayo Clinic. Mayo Clinic. February 26, 2020. Web.
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