Fetal Heartbeat Explained

Fetal Heartbeat: Timing, Detection, and What It Means when Navigating Your Options

When does the fetal heartbeat start?

One of the key stages in early human development is when the heartbeat starts. This article gives a quick rundown on early human growth, pinpoints when the heart begins to beat, and explains why it’s important for you and your medical team. We’ll also clear up some terms that might be confusing, like “embryo” and “fetus,” or “fetal age” and “gestational age.”

For more details on these and related topics, click on the drop-down links below.

Phase 1

Human life begins when a woman’s egg is fertilized by a man’s sperm. This process offertilization—also referred to as conception—usually happens in the fallopian tube and creates a single-celled human being known as a zygote. This new life starts growing right away, quickly dividing to become a multi-celled blastocyst that, within a week after conception, implants into the woman’s uterine lining. It also develops the placenta, which transfers nutrients and oxygen from the woman to the developing baby and helps clear the baby’s waste products back to the mother for elimination.

From implantation until the end of the eighth week of pregnancy, the

developing baby is called an embryo; from the ninth week until birth, it’s called a fetus.

Phase 2

About 18-22 days after conception (or around 5-6 weeks into pregnancy), the human heart starts beating. At this stage, the tissues that will eventually become a fully formed heart start to pulse, creating heartbeats that can be detected by ultrasound.[1] The fetal heart is fully formed by the end of the tenth week of pregnancy.[2]

Phase 3

In weeks 5-6 of pregnancy, the embryonic heart beats roughly 110 times per minute. As the heart

keeps developing through the ninth and tenth weeks, the fetal heart rate generally rises to 140-

170 beats per minute, then usually levels out between 110 and 160 beats per minute.3

While life itself begins at conception, a detectable heartbeat at about 18-22 days is a key sign of

a healthy pregnancy. It also significantly boosts the chances that the baby will make it to

childbirth.

Phase 4

While life itself begins at conception, a detectable heartbeat at about 18-22 days is a key sign of a healthy pregnancy. It also significantly boosts the chances that the baby will make it to childbirth.

When does the fetal heart start to beat, and how is it detected or measured?

The fetal heart starts to beat about 18-22 days after conception (or around 5-6 weeks into

pregnancy). It is detected or measured using a fetal ultrasound.

“During a fetal ultrasound,” the Mayo Clinic reports, “a device called a transducer is placed on

the pregnant person’s belly. Or in some cases, it may be placed in the vagina or on the area

between the vagina and the anus. Sound waves are translated into a pattern of light and dark

areas.” This pattern “creates an image of the fetus on a screen,” which can then be interpreted by

a trained medical professional.

Depending on how far along the pregnancy is, medical professionals might use different types of

ultrasound scans to detect or measure the fetal heartbeat. In the first trimester, up to around 11

weeks, they usually use a transvaginal scan to find the heartbeat. After this, from about the

twelfth week onward, it’s more common to use a transabdominal scan, though either type of scan

may be used at any point during pregnancy. Finally, according to the Mayo Clinic, doctors keep

an eye on the baby’s heartbeat during labor in one of two ways:

Auscultation: This involves holding a special stethoscope or a Doppler transducer against the pregnant person’s abdomen and listening for the fetal heartbeat. The doctor may do this at specific times during labor.
Electronic fetal monitoring: This involves using specialized internal or external equipment to measure the heart rate in response to contractions. It provides an ongoing reading, which the doctor can check at set times. [5]

Why is a fetal heartbeat important?

Detecting a heartbeat around the fifth or sixth week of pregnancy is a major milestone for a developing baby. It shows things are going smoothly. The heart forms early because the baby needs a steady supply of oxygen-rich blood to survive.[6]

Also, research shows that hearing a heartbeat at 6-8 weeks means the baby has a very good chance of making it to birth.[7] As Healthline reports, a study of 325 women with previous miscarriages found that if a heartbeat is detected at 6 weeks, there’s a 78 percent chance of the pregnancy continuing. By 8 weeks, that chance jumps to 98 percent, and by 10 weeks, it’s 99.4 percent.[8]

In short, the development of the heart and detecting its beat around weeks 5-6 is a good sign that the baby is growing well and is likely to make it to birth.

What is an ultrasound and how is it performed?

As the Cleveland Clinic explains,

A prenatal or pregnancy ultrasound uses sound waves to create a picture of your baby on a screen. Pregnancy care providers use it to check on the health of your baby and detect certain pregnancy complications. Most people have two ultrasounds during pregnancy, but you may have more if your provider feels it’s medically necessary.[9]

Doctors might use a prenatal ultrasound for several reasons, like:

Confirming you’re pregnant
Checking for ectopic pregnancy, molar pregnancy, miscarriage, or other early pregnancy issues.
Determining your baby’s age and due date.
Checking your baby’s growth, movement, and heart rate.
Looking for multiple babies (twins, triplets, etc.).
Examining your pelvic organs like your uterus, ovaries, and cervix.
Checking the amount of amniotic fluid.
Checking the location of the placenta.
Checking your baby’s position in your uterus.
Detecting problems with your baby’s organs, muscles, or bones.[10]

As mentioned before, it’s common to have at least two ultrasounds throughout pregnancy. Doctors can see an embryo and its heartbeat on an ultrasound as early as 5-6 weeks into pregnancy,[11] so you might have your first routine scan around the seventh or eighth week. This early scan confirms pregnancy, checks for multiple babies, measures the baby’s size, and helps confirm the due date.[12]

Later, around 18 to 20 weeks, you might have an “anatomy ultrasound” or “20-week ultrasound.” During this ultrasound, your doctor “can see your baby’s sex (if your baby is in a good position for viewing their genitals), detect birth disorders like cleft palate or find serious conditions related to your baby’s brain, heart, bones or kidneys.”[13]

Additional ultrasounds may be ordered if needed, but if everything is going well, the 20-week ultrasound might be your last one during pregnancy.[14]

Pregnancy ultrasounds can be done in two ways. Transvaginal ultrasounds, usually done earlier in pregnancy, involve placing a “wand-like device” inside your vagina. Transabdominal ultrasounds, typically used after 12 weeks, involve placing a transducer on your belly. In both types of ultrasound scans, sound waves create images of the developing baby.[15]

Ultrasound tests are typically pain-free and very low-risk for both the baby and the mother. Because it doesn’t use radiation (like X-rays), “ultrasound is the most widely used medical imaging method for viewing an unborn baby during pregnancy.”[16]

What are the different types of ultrasound scans? What is a Doppler ultrasound and when is it used?

There are actually a few different kinds of ultrasound scans. The standard one is 2D, which is the most common. Then there are 3D and even 4D scans. Here’s how the Cleveland Clinic explains them:

For ultrasounds during pregnancy, the traditional ultrasound is a two-dimensional (2D) image of the fetus. 2D ultrasound produces outlines and flat-looking images, which allows your healthcare provider to see the fetus’s internal organs and structures.

Three-dimensional (3D) ultrasound allows the visualization of some facial features of the fetus and possibly other body parts such as fingers and toes. Four-dimensional (4D) ultrasound is 3D ultrasound in motion. Providers rarely use 3D or 4D fetal ultrasound imaging for medical purposes, though it can be useful in diagnosing a facial or skeletal issue. They do, however, use 3D ultrasound for other medical purposes, such as evaluating uterine polyps and fibroids.[17]

Whether 2D, 3D, or 4D, a standard ultrasound shows images of the baby but doesn’t measure blood flow.[18] For that, there’s a special type of scan called a Doppler ultrasound. This type uses sound waves to check the movement of blood in vessels, specifically red blood cells. These sound waves bounce back to the ultrasound probe on the mother’s abdomen when they hit the red blood cells. By comparing the sound waves sent out to those that come back, an image is created.[19] This image shows how well blood is flowing through the baby’s blood vessels.[20]

During pregnancy, a Doppler ultrasound might be used if the baby is smaller than expected for that stage, if the mom has high blood pressure (preeclampsia), or if she has sickle cell disease. It’s usually done later in pregnancy.[21]

When does pregnancy begin and how long does it last?

We usually think of pregnancy as lasting around nine months, divided into three trimesters: the first three months, the middle three months, and the last three months.

But actually, pregnancy is measured in weeks.

According to the Cleveland Clinic, “[a] full-term pregnancy is 40 weeks, or 280 days.”[25] It starts from the first day of a woman’s last menstrual period (LMP), which is about two weeks before conception actually happens.[26] The baby’s “gestational age” is based on the date of the woman’s LMP,[27] which is also what we refer to when we talk about how far along a woman is in her pregnancy. So, pregnancy officially begins on the first day of a woman’s last period.

What can be a bit confusing is that since conception is assumed to happen two weeks after a woman’s last period, there’s a two-week period where the woman’s body is “getting ready”[28] for pregnancy even though she’s not technically pregnant yet. The American College of Obstetricians and Gynecologists (ACOG) explains it like this:

A normal pregnancy lasts about 40 weeks from the first day of your last menstrual period (LMP). Pregnancy is assumed to start 2 weeks after the first day of the LMP. So an extra 2 weeks is counted at the beginning of your pregnancy when you are not actually pregnant. The 40 weeks of pregnancy includes those extra 2 weeks.[29]

What is the difference between an “embryo” and a “fetus”?

As explained in the Cleveland Clinic’s article “Fetal Development,” the growth of a baby before

birth has three main stages: the germinal, embryonic, and fetal stages.22

The germinal stage starts when a sperm and egg meet in a woman’s fallopian tube—a process

known as fertilization, or conception. The result is a new, single-celled23 human called a zygote.

This zygote starts growing right away, its cells dividing quickly until it becomes a multi-celled

blastocyst. About a week after conception, the blastocyst implants into the lining of the woman’s

uterus.

Now, about three weeks into pregnancy (or one week after conception), the blastocyst is called

an embryo. This term is used until the end of the eighth week of pregnancy (six weeks after

conception).

It is during this stage, at about five to six weeks into pregnancy, that the embryo’s cardiac

activity begins. At this point, the cells that will form the heart start to pulse, marking the

beginning of the fetal heartbeat.

From the start of week 9 of pregnancy until birth, the baby is called a fetus. During this time:

– The baby’s major organs and body systems keep growing and maturing.

– Things like fingernails, eyelashes, and hair start to grow.

– The baby can move its limbs, although you might not feel it until 20 weeks.

– Most of the baby’s growth in both weight and length happens during this stage.24

In short, the difference between “embryo” and “fetus” is simply a matter of stages in a baby’s

development before birth. “Embryo” and “fetus” refer to different stages in the life of a baby

before it is born, just like “infant,” “toddler,” “child,” “teenager,” “adult,” and “senior citizen”

refer to stages after birth.

What is the difference between “fetal age” and “gestational age”?

Fetal age has to do with how old the embryo or fetus is, starting from the moment of

fertilization. In other words, it’s the actual age of the developing baby from the time it was

conceived. So, if you hear “6 weeks of fetal age” or “6 weeks after fertilization,” it means the

baby has been growing for six weeks since conception.

Gestational age, on the other hand, is calculated based on the first day of the woman’s last

menstrual period (LMP). This method helps determine when the pregnancy started and the

estimated due date. Since fertilization usually happens about two weeks after the LMP, the

pregnancy timeline includes these first two weeks. So, if a woman is said to be eleven weeks

pregnant (gestational age), the fetus is actually about nine weeks old (fetal age).

In practical terms, when a woman says she is “9 weeks along” in her pregnancy, she’s talking about the gestational age of her fetus. Saying a woman is in her ninth week of pregnancy means it's been nine weeks since the first day of her LMP, and that the gestational age of her fetus is nine weeks as well.

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