Pregnancy Information Center

Information, Symptoms, Treatments and Resources


Your First Trimester - Week 4


Week 4


What's Happening To Your Body

At week 4 of your pregnancy, implantation is still taking place. You'll begin to produce higher levels of the hormones estrogen and progesterone, which will cause many of the changes in your body throughout the rest of the pregnancy.

At about 6 to 12 days after fertilization, your hCG levels will be high enough to be detected in a home pregnancy test. If the test is positive, schedule an appointment with your healthcare provider to confirm the results. You may also experience the typical signs of pregnancy, including fatigue, frequent urination, breast pain and morning sickness.


What's Happening To Your Baby

Your baby is the size of a mustard seed. The cells in the embedded blastocyst begin to differentiate, or specialize, into distinct layers which later undergo further differentiation. One such layer undergoes a process known as gastrulation in which it further differentiates into three layers that will form the baby’s organs and tissues (the ectoderm, endoderm, and mesoderm). At this point, your baby is roughly the size of a poppy seed.

In the late stages of week 4 or early stages of week 5, tiny arms and legs, called buds, begin to develop, although they are not yet visible.


Read This Next: Week 5

In Case You Missed It: Week 3


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