Q: When is the human heart fully developed, and when are heartbeats audible during pregnancy?
A: A developing heart has all of its primary structures after about nine weeks of pregnancy. Some forms of ultrasound can detect cardiac activity in an embryo in the sixth week, but a heartbeat wouldn’t be audible until about 10 weeks on a Doppler fetal monitor.
FULL QUESTION
When does an ACTUAL heart become fully functioning and formed?
When do doctors actually hear a heartbeat?
I was confused after reading a Popular Science article along with information shared by MayoClinic. Thank you for any information that you can provide.
FULL ANSWER
In the past four months, five states (Kentucky, Mississippi, Ohio, Georgia and Louisiana) have passed laws that ban abortion once a heartbeat becomes detectable during a pregnancy, which is generally possible by the sixth week of gestation, or the fourth week after conception.
For this reason, abortion rights advocates such as Planned Parenthood refer to these measures as six-week abortion bans. Others, including proponents, call them “heartbeat” and sometimes “fetal heartbeat” legislation. All of the state laws allow abortion if the pregnant woman’s life or physical health is severely endangered, but only some make exceptions for fatal chromosomal anomalies in the fetus or in cases of rape or incest.
“Fetal heartbeat” is more of a legal term than a medical one, as some of the state laws specifically define a fetus as existing from the moment of conception. In medicine, however, a fetus doesn’t exist until eight weeks after fertilization, or at the end of the 10th week of pregnancy, when all the major organ systems have begun to form. Before then, the correct term is an embryo.
While none of the laws are currently in effect — and some have already been blocked in court — news coverage of the legislation has led to confusion. A Popular Science article, for instance, described the heartbeat at six weeks as a “rhythm a doctor can pick up on an ultrasound,” stating that “it isn’t a heartbeat, because the embryo has no heart.”
The American College of Obstetricians and Gynecologists also has said in a statement, “What is interpreted as a heartbeat in these bills is actually electrically-induced flickering of a portion of the fetal tissue that will become the heart as the embryo develops. Thus, ACOG does not use the term ‘heartbeat’ to describe these legislative bans on abortion because it is misleading language, out of step with the anatomical and clinical realities of that stage of pregnancy.”
At the same time, many online medical websites, including the Mayo Clinic, do refer to the heart and its beating early-on in pregnancy. And plenty of medical textbooks use the words “heart” and “heartbeat” to refer to the embryo’s developing heart.
These sources don’t include many details on how heart development occurs, so it’s no surprise our reader was confused.
Precise answers to these questions depend on an individual’s definitions, but the heart doesn’t have all of its major components until the beginning of the 10th week of pregnancy. Beating occurs earlier, at the very end of the fifth week or at the start of the sixth week, when the heart is immature and lacks most of its identifiable features, including its iconic chambers. The heartbeat shows up around this time or shortly after as a flickering motion on an ultrasound. Only later, after the 10th week or beyond, is a heartbeat audible with a Doppler fetal monitor.
We’ll walk through what is known about heart development to provide more context. We’ll also share the various ways heartbeats are monitored throughout a pregnancy, and the medical significance of early heartbeat detection.
But first, a quick primer on how pregnancy weeks are calculated, because there are multiple ways of counting this time. One way is to count from fertilization, or when sperm meets egg. Fertilization dates, however, are not usually known, so pregnancy is more often measured from the last menstrual period. This way of counting, which is also termed gestational age, includes approximately two weeks prior to conception. As a result, a six-week pregnancy by this measure includes only about four weeks of actually being pregnant. A normal pregnancy lasts approximately 40 weeks.
The first sign of pregnancy is often a missed period, which would occur, on average, 14 days after conception, or at four completed weeks of pregnancy. Many women, however, do not have regular periods, so they may not know they are pregnant until later.
To be consistent, we will refer to weeks of pregnancy as measured from the last menstrual period, but will also give days since conception in some cases, as this is a standard measurement in embryology.
Heart Development 101
We consulted multiple embryology textbooks and several scientific reviews and research papers to learn about heart development, and also spoke with a pediatric cardiologist and researcher, Colin Phoon of New York University Langone Health.
A good rule of thumb, according to Phoon, is that heart development occurs over a four-week period, starting in the sixth week of pregnancy. Before the sixth week, he said, “There is no heart; there’s nothing beating.”
Textbooks and scientific papers don’t always give the same exact dates for each milestone, and the timing is often extrapolated from animal studies. So scientists may eventually learn more and revise their timelines. But a general consensus is that around 21-23 days after conception, two groups of cells that form a horseshoe shape fuse together to form a tiny, hollow tube. This tube is known as a heart tube, and initially is very simple. It’s straight, a bit like a straw, and doesn’t have any chambers that are typical of a developed heart.
Very soon after the tube forms, some cells of the tube begin to spontaneously contract, creating the first heartbeat, although the heart tube may not pump blood for another day or two. Textbooks and papers also peg this to approximately 21-23 days after fertilization, or what would be five completed weeks of pregnancy, or a few days into the sixth week.
Over the next several days, the heart tube elongates and loops, bending and twisting into a more recognizable heart shape. “It does a bunch of funny convolutions,” said Phoon. “It has to curve on itself, it has to loop. It eventually has to become four chambers and four valves,” he said, adding that the veins and arteries — including the aorta, which exists as a series of vessels early-on — must also develop.
The process of forming separated heart chambers begins around 28 days, or six completed weeks of pregnancy, as tissue starts to form to divide up the tube. The chambers include the upper left chamber, or left atrium, which after birth will collect oxygen-rich blood coming from the lungs, as well as the right atrium, which collects oxygen-depleted blood coming from the rest of the body. The two lower chambers, or ventricles, perform the reverse functions, and pump blood back out to either the lungs or the rest of the body. In the embryo and fetus, oxygen comes not from the lungs, but from the mother, via the placenta.
The last key elements of the heart that begin to develop are the valves, which are important flaps between the upper and lower chambers and between the ventricles and the major arteries. These make sure blood moves only in one direction through the heart.
Chamber and valve formation take about three weeks to wrap up. Based on 3D imaging of human embryos, scientists are able to identify all of the major structures after nine weeks and one day of pregnancy, when the entire process is largely complete.
That’s not to say that heart development doesn’t continue. As one cardiology textbook explains, “The heart is the first functional organ in the mammalian embryo; however, its full development spans the whole intrauterine period and is finished only in the postnatal period.” There is, for example, some fine tuning of the valves later in pregnancy, and of course, the entire heart continues to grow. Cardiac muscle also changes its composition and structure over time, including a shift to a more mature helical organization well into the second trimester. But the majority of the developmental action occurs within those first nine to 10 weeks.
Heartbeat Detection
Heartbeats are first detectable with a transvaginal ultrasound, usually after six completed weeks of pregnancy, but also sometimes during the sixth week.
The real-time scan, however, doesn’t provide expecting parents with an audible heartbeat.
As Erika Werner, a maternal fetal medicine physician at Brown University and a Society for Maternal-Fetal Medicine board member, explained, the ultrasound at this stage is visual-only.
“You can see a little flicker,” she said. “That’s it.”
The ultrasound is picking up on the slight movement of the developing heart while it beats, as high frequency sound waves are sent out from the machine, get reflected back when they hit different kinds of fluids and tissue, and are used to form an image.
Transvaginal ultrasounds, or ultrasounds that use a probe inserted into the vagina, are closer to the growing embryo, so they’re more likely to detect cardiac motion before six weeks. The more standard abdominal ultrasound, Werner said, can be used after six to eight completed weeks.
But even then, it may depend on a variety of factors. With both types of ultrasound, Werner said, whether or not a heartbeat is detectable can depend on the patient’s weight, the presence of scar tissue, and the position of the uterus and fetus. The more tissue you have to penetrate with the sound waves, she said, the less likely the ultrasound will be able to pick up a heartbeat.
Even the quality of the ultrasound machine matters. In places with poor machines, Werner noted, the flicker might not be apparent until weeks later.
What most parents experience as the first audible “heartbeat” during pregnancy comes from a device known as an acoustic Doppler or a Doppler fetal monitor. The machine is handheld and also uses ultrasound waves, taking advantage of the Doppler shift.
Technically, the heart sounds that the machine produces are not the actual sound of the fetus’s heart beating. As a product manual for one such device explains, the sound is the amplified version of the difference between the transmitted and received signals. “It is important to remember that the sound you hear is an artificial sound, the frequency (pitch) of which is proportional to the velocity of the moving target,” the manual reads. “It is not the real sound made by blood rushing through an artery or vein, or movement of the fetal heart.”
Werner said she doesn’t usually try an acoustic Doppler on a patient until at least 10 weeks, and on an obese patient, it still may not work until after 12-14 weeks.
Doppler ultrasounds, which are distinct from the handheld monitors, can also provide audio and visual confirmation of a heartbeat during pregnancy, including more detailed evaluations of fetal blood flow. The American Institute of Ultrasound in Medicine, however, discourages the use of such machines in the first trimester.
To hear a true heartbeat, expecting parents can turn to a stethoscope — if they are willing to wait. According to the 25th edition of “Williams Obstetrics,” a standard stethoscope successfully picks up fetal heartbeats in 80% of patients after 20 weeks, and in all patients after 22 weeks. But Werner said that in her practice, most OB/GYNs don’t even carry stethoscopes. “All the rooms,” she said, “have acoustic Dopplers.”
What Early Heartbeat Detection Means
In medicine, the detection of a heartbeat is important because it signals that the embryo or fetus is alive and that the pregnancy is likelier than not to continue.
According to the 7th edition of “Obstetrics: Normal and Problem Pregnancies,” once there is identifiable cardiac activity, only about 2-3% of pregnancies in low-risk women will fail.
The miscarriage rate, however, can be much higher in other groups. One study, for instance, found that in women 36 years or older receiving fertility treatment, 16% of pregnancies ended in miscarriage even after confirmation of cardiac activity in the eighth week.
A heartbeat, then, is no guarantee of a continued or healthy pregnancy, although it is a requirement for one.
A heartbeat, it should also be mentioned, does not necessarily mean that a fetus is viable, or capable of surviving outside the womb. Older fetuses might be, but after just six weeks, no embryo is capable of living on its own, even with medical assistance. According to UpToDate, an online medical resource for physicians, many infants will live if delivered after 26 weeks of pregnancy, but almost none do if delivered before 22 completed weeks.
Viability is an important legal milestone with respect to abortion. In 1992, the Supreme Court held in Planned Parenthood v. Casey that women have the right to abortion before fetal viability, and that states cannot legally impose an “undue burden,” or create “substantial obstacles in the path of a woman seeking an abortion,” before that time.
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