Since you found out you were pregnant, one milestone you've likely eagerly awaited is hearing the thump-thump of your baby's heartbeat. It's one of the most reassuring sounds there is.
And though it probably will sound the same to you from checkup to checkup, there are big changes happening to the heart and circulatory system every week of your pregnancy!
When does your baby have a heartbeat?
By week 5 of pregnancy, the cluster of cells that will become your baby's heart has begun to develop and pulse. If you have a first trimester ultrasound (around or after week 6 of pregnancy), your practitioner or a trained sonographer will check on this embryonic cardiac activity.
The ultrasound will also confirm your estimated due date, and how many babies you're carrying.
The chambers of your baby’s heart will have developed enough to be seen more clearly on an ultrasound by weeks 17 to 20 of pregnancy.
When can you hear a baby’s heartbeat?
You may see (and/or hear) cardiac activity for the first time from week 6 of pregnancy or later if you have an ultrasound at one of your early prenatal appointments, though the timing of when it can be detected can vary a bit.
Can't hear the heartbeat yet? Don't worry. It likely just means that your shy guy is hiding in the corner of your uterus or has his back facing out, making it trickier to pick up on an ultrasound.
At your next appointment, your practitioner will check to make sure everything is okay. Chances are, you'll be able to hear some comforting sounds then.
Later on, at your 20-week ultrasound (also called the level 2 ultrasound), you'll hear — and see — your baby's heartbeat.
More About Fetal Development
Normal fetal heart rate
By 6 weeks, the embryonic heart cells will pulse about 110 times a minute. In just two more weeks, the fetal heart system will contract at a pace of around 150 to 170 times a minute. That's about twice as fast as yours!
By week 9 or 10, the rate will hover around 170 beats per minute — and then slow from here on out. At around week 20, it'll go down to about 140 beats per minute.
During labor, a typical fetal heart rate can span from 110 to 160 beats per minute, although brief variations outside this range can occur for a variety of (often perfectly normal) reasons.
When can you hear your baby’s heartbeat with Doppler?
You'll most likely hear fetal cardiac activity with a Doppler at around the 15-week mark. Your doctor or midwife will place this handheld ultrasound device on your belly to amplify the pitter-patter of the heart.
Keep in mind that experts including the Food and Drug Administration (FDA) warn against using at-home fetal Dopplers unless you're under the supervision of a medical professional.[1]
That's in part because these devices aren't as sophisticated as the ones that doctors use, so they may not pick up a fetal heartbeat — leading to an unnecessary scare. Plus, it can be hard to use an at-home Doppler properly without training. You could, for example, mistake your own heartbeat for your baby's.
How your baby's heart and circulatory system develop
Embryonic heart development starts early on in pregnancy, and your baby's ticker continues to change even after birth as he adjusts from the womb to the outside world.
First trimester development
By week 4, a distinct cluster of cells has formed inside your embryo, which will soon develop into your baby's heart and circulatory (blood) system. At week 5, the preliminary structures that will become your baby's heart begin spontaneously pulsing.
In these early stages, it resembles a tube that twists and divides to eventually form the heart and valves (which open and close to release blood from the heart to the body). Precursor blood vessels also begin to form in the embryo during the first few weeks.
Second trimester development
By 17 weeks, the fetal brain has begun to regulate the heartbeat in preparation for life in the outside world. (Up until this point, the cardiac electrical activity occurred spontaneously.)
Capillaries also form at an exponential rate during the second trimester. These teeny-tiny blood vessels deliver oxygenated blood to the tissues in your baby's body and then recycle deoxygenated blood back into the circulatory system.
Between 17 and 20 weeks, the heart chambers have developed enough to appear more clearly on an ultrasound. During your second trimester anatomy scan, your doctor will check the structure of your baby's heart and look for any congenital heart defects.
If your doc needs a better listen and view of your baby's heart, he or she may recommend a fetal echocardiogram between 18 and 24 weeks of pregnancy. You'll want to make sure you get one if you have a family history of congenital heart defects, or if you have diabetes, phenylketonuria or an autoimmune disease.
Third trimester development
Baby's circulatory system will continue to grow slowly and steadily during the last trimester, so that by 40 weeks, it's ready for its debut outside the womb.
How the fetal heart works
While the fetal circulatory system develops rapidly throughout pregnancy, it actually works quite differently in utero than it does once your baby is born.
Since babies don't breathe in utero, the lungs don't actually function before birth. Until then, your baby’s developing circulatory system relies on the umbilical cord for a steady supply of oxygen- and nutrient-rich blood.
Umbilical arteries and veins transport what your baby needs from you to him, then carry unoxygenated blood and waste products back to you for removal.
Most of the physical differences between the fetal and adult heart serve the purpose of directing blood away from baby's lungs, which simply aren't needed in utero. The fetal heart has:
- Three shunts. These shortcuts direct blood away from the lungs and the liver.
- A ductus arteriosus. This fetal artery connects the pulmonary artery (which will eventually bring blood from the heart to the lungs) and the aorta (which will bring blood from the heart to the body).[2] It detours blood away from the lungs in utero.
- A foramen ovale. This for-the-womb-only opening between the upper chambers of the heart again shunts blood away from the lungs.[3]
Once your baby is born, all these fetal differences start to go away or disappear completely. When the umbilical cord is cut, his lungs take in air, the fetal circulation system is switched off, and the shunts begin to close. All systems are go!
From the time you get your positive pregnancy test, your baby's heart is developing and changing rapidly. To help ensure that his ticker gets the healthiest start possible, take your prenatal vitamin throughout pregnancy, quit smoking or vaping (if you currently light up), and avoid alcohol and recreational drugs. And enjoy these milestones along the way. Soon enough, you'll be able to cradle your baby in your arms and hear his little heart beating next to yours.
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