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When you find out you're pregnant, hearing your baby's heartbeat is probably one of the first milestones you anticipate. The good news is that you don't have to wait long. A baby's heart is up and running early on because it's needed to deliver oxygenated blood and nutrients to developing organs.
If you have a first trimester ultrasound around 8 weeks, you may be able to hear (and see) the heartbeat. Otherwise, you'll probably first hear it at a prenatal care visit when your provider checks you with a fetal Doppler, a small handheld device that's pressed against your belly. You may have this as early as 10 weeks, but it's more common at 12 weeks.
Early development of the heart
During the first few weeks of pregnancy, your tiny embryo is shaped like a flat disk. By 5 weeks, two tubes that will become the heart have formed. The two tubes fuse together and blood flows through this tubular "heart" as it begins to beat.
Between 6 and 7 weeks, the heart tube twists and bends into an S shape. The bottom of the tube moves up and toward the back and will form the two upper heart chambers (atria). The top of the tube will form the two lower heart chambers (ventricles) as well as the large vessels that transport blood from the heart.
By 9 weeks, the four chambers of the heart are formed.
Development of blood and blood vessels
Blood begins to form as early as 4 weeks in little structures called blood "islands" that soon give rise to individual blood cells and blood vessels, including the aorta. The aorta is the large blood vessel that delivers oxygenated blood from your baby's heart to its developing organs.
Oxygenated blood comes to the baby's heart from the placenta through veins in the umbilical cord and deoxygenated blood returns to the placenta through umbilical arteries.
The liver then starts producing blood cells, followed by other organs, like the spleen. Once your baby's bones are developed enough, blood cells will primarily be produced in the bone marrow, which becomes the main site where blood is made for the rest of her life.
Bypassing the lungs until birth
Before birth, your baby has a small opening in the wall between the two atria so blood can flow from the right to left atrium. (A flap-like valve prevents blood from flowing the other way.) This opening allows blood to bypass your baby's lungs, which aren't necessary until your baby is born because blood from the placenta supplies oxygen for now.
This hole normally closes after your baby is born and takes her first breath. In most babies, it's completely sealed within a few months.
What does the fetal heartbeat sound like?
If you expect to hear the familiar steady "lub-dub" sound of a human heartbeat, you're in for a surprise. Many women describe the sound of their baby's heartbeat as similar to the thunder of galloping horses. That's because your baby's heart beats about twice as fast as yours does. (During your second trimester, it averages about 140 beats per minute.)
What you can do during pregnancy
Getting regular exercise during pregnancy is great for your heart, and evidence increasingly suggests that it's good for your baby's heart as well. In a study of women who exercised regularly during pregnancy (30 minutes, three times a week), their fetuses had a lower heart rate and greater heart rate variability compared with the fetuses of women who didn't exercise at all. (Both are signs of healthy heart function.) These benefits also continued after the babies in the study were born.
Key milestones in fetal heart development
|5 weeks||Two heart tubes have formed in the embryo. The two tubes fuse and blood flows through this tubular "heart" as it begins to beat.|
|6-7 weeks||Walls begin to form that will divide the heart into four chambers.|
|8 weeks||You may be able to see and hear your baby’s heart beat in an ultrasound exam.|
|9 weeks||The four chambers of the heart are formed.|
|10 to 12 weeks||You may be able to hear your baby's heart beat with a Doppler device.|
|Birth||The opening between the atria, which allows blood to bypass the lungs in utero, closes.|