When does the heart develop in a fetus?

When does your baby develop a heartbeat?

A baby's heartbeat (at this point a fluttering of cells) starts as early as to 4 weeks after conception or 6 weeks after the first day of the last menstrual period.

When you find out you're pregnant, seeing your baby's heartbeat may be one of the first milestones you anticipate. You don't have to wait long. A baby's developing heart is up and running quickly because it's needed to deliver oxygenated blood and nutrients to other developing organs.

When you're 6 weeks pregnant, however, the heart isn't fully developed. For this reason, the American College of Obstetrics and Gynecologists (ACOG) defines the fluttering of cells in the embryonic heart tube as "cardiac activity" rather than a heartbeat.

When can you see your baby's heartbeat?

Around 4 weeks after conception, or 6 weeks after your last menstrual period, the fast flickering of the embryo's developing heart tube can be seen on ultrasound.

If you have a first trimester ultrasound (perhaps because you've had a previous pregnancy complication), it's possible to see cardiac activity this early. This ultrasound can also be used to screen for heart problems, which may be important if there's a family history of congenital heart problems.

If at your first ultrasound your provider can't see the future heart pulsating, it may simply be that your dating is off and it's too soon to see the heart motion. (This is common!) If there's a "fetal pole" (visible embryo) and no heart motion, it's possible that you have a "missed miscarriage" or blighted ovum. This means that the fertilized egg isn't developing. Your obstetrician or midwife will talk with you about what your individual ultrasound means and what your next steps are.

When can you hear your baby's heartbeat?

If you don't have an early ultrasound, you'll probably first hear your baby's heartbeat at a prenatal care visit between 10 and 16 weeks. Your provider will use a fetal Doppler, a small handheld device that's pressed against your belly with some gel. Sometimes the positioning of your uterus, baby, and the shape of your belly can make it hard to find the heartbeat.

If you expect to hear the familiar steady "lub-dub" sound of an adult heartbeat, you're in for a surprise. Many women describe the sound of their baby's heartbeat as similar to galloping horses. That's because your baby's heart beats much faster than yours does, averaging between 110 and 160 beats per minute. (A normal resting heart rate for adults is 60 to 100 beats per minute.)

Once you're about 20 weeks pregnant, your baby's heartbeat can be heard with a stethoscope.

How to support your baby’s heart before and during pregnancy

It's important to do everything you can to stay healthy before and during pregnancy. But it's not a guarantee that nothing will go wrong. Some things – like some congenital heart defects – are largely out of your control. Some congenital heart defects may be related to a disease the mother has, such as diabetes. In other cases, heart problems may run in families. In most cases, though, doctors don't know why specific congenital heart defects happen.

Some specific things you can do:

• Avoid smoking and secondhand smoke. A number of studies have shown a strong link between cigarette smoke and congenital heart (and other) defects.
• Be careful with medications. Talk with your healthcare provider before trying to conceive if you're on regular medications. Some medications – such as ACE inhibitors, the acne medication isotretinoin, lithium (a psychiatric medication), and warfarin (an anticoagulant) increase the risk of heart defects. You and your doctor may decide together that controlling your underlying health problems with this specific medication is worth the small risk of a fetal heart defect. However, there are often safer alternatives, so explore options with your provider.
• Manage any chronic health conditions. If you have diabetes before becoming pregnant, for example, take steps to control your blood sugar before and during pregnancy. Out-of-control pre-pregnancy diabetes is one of the leading causes of congenital heart defects.
• Don't drink alcohol. Studies have shown that alcohol exposure during pregnancy is significantly associated with heart defects. There's no known safe amount of alcohol during pregnancy.
• Get your rubella vaccination, if you need one, before becoming pregnant. When you see your doctor for a preconception visit, they can check to see if you're still immune from your childhood MMR vaccine. Occasionally, immunity wanes and women benefit from a pre-pregnancy MMR booster. (You shouldn't get the vaccination during pregnancy.) Getting rubella during pregnancy can affect the development of your baby's heart.
• Let your doctor know about any family history of congenital heart problems. They can order special ultrasounds to monitor the development of your baby's heart.

Key milestones in fetal heart development

Your baby's heart develops from early in pregnancy through – and even after – birth.

5 weeks pregnant

Two heart tubes have formed in the embryo. The two tubes fuse and blood flows through this tubular "heart" as it begins to beat. The sections of this tube will go on to form all the structures of the future heart.

At first, all the blood flows into the bottom of the heart, and contractions propel the blood to the top of the tube.

6 to 7 weeks

The heart tube twists and bends into an S shape to begin forming all four chambers. The bottom of the tube moves up and toward the back and will form the two upper heart chambers (atria). The middle of the tube will form the two lower heart chambers (ventricles). Some congenital heart defects happen when the heart doesn’t twist or divide normally. Walls begin to form that will divide the heart into the four chambers. Each chamber has an entrance and exit for blood flow.

You may be able to see your baby's heartbeat in an ultrasound exam.

8 weeks

The valves between the atria and ventricles have formed.

9 weeks

The valves between the ventricles and the aorta and pulmonary artery have formed. (The aorta is the large blood vessel that delivers oxygenated blood from your baby's heart to its developing organs.) The four chambers of the heart are fully formed.

10 to 12 weeks

The baby's brain starts regulating the heartbeat, which has been beating independently of the nervous system up until now.

You may be able to hear your baby's heartbeat with a Doppler device.

16 to 20 weeks

If you're at high risk, a fetal echocardiogram or a comprehensive or level II ultrasound can be performed to screen for problems with your baby's heart.

25 weeks

The capillaries (smallest blood vessels) are forming and filling with blood.

Birth

The opening between the two atria (called the foramen ovale), closes when your baby is born and takes their first breath. This opening allowed blood to bypass your baby's lungs, which weren't necessary until your baby was born because blood from the placenta supplied oxygen until then. In most babies, the opening is completely sealed within a few months after birth.

The ductus arteriosis also closes at birth. This is a connection between the right heart circulation and the aorta that also allowed oxygenated blood to bypass your baby's lungs before birth. After their first breath, your baby makes the amazing transition to getting oxygen from their own lungs instead of from you and the placenta.

The baby's circulatory system is different after birth than before birth, so sometimes congenital heart defects aren't apparent until a few hours or days of life. This is one of the many things your baby's pediatrician will be looking for on those newborn exams.

How do the blood and blood vessels develop?

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.

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. Other organs, like the spleen, follow. 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 their life.

It's fascinating to learn about your baby's development in the womb. Keep track of where your baby is, week by week, with our slideshow on fetal development.