✓ When does the fetus start to feel the heartbeat?

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The most exciting moment of the first gynecological examination is being able to hear the baby's heartbeat. The doctor makes you lie down on the bed and with the ultrasound shows you a small green bean and then there it is, the flicker. That's the heart of your little one, beating very fast. But since when can it be heard and how much is it beating?

Fetus heartbeat: when it starts to feel

Fetal heartbeat can be detected as early as 34 days (just under 6 weeks) of gestation with a good quality high-frequency transvaginal ultrasound, such as a crown croup length (CRL) of just 1-2mm.

By 6 weeks, the embryo's heart - which now beats 110 times per minute - has four empty chambers, each with an entrance and an exit to allow blood to flow in and out of them. In just another two weeks, that number will jump to 150-170 beats per minute.

With all this growth, probably you will be able to hear the fetal heartbeat for the first time around 9-10 weeks of pregnancy, although the exact time may vary. At this point it will be about 170 beats per minute, a speed that will slow down from here on out. To hear this, the doctor or midwife will place a portable ultrasound device called a Doppler on the abdomen to amplify the sound.

Fetus heartbeat: how much it beats

The heartbeat is first visible on ultrasound around 6 weeks of gestation. The fetal heart rate at this stage is usually between 100 and 120 beats per minute (bpm).

A normal fetal heart rate (FHR) usually ranges from 120 to 160 beats per minute (bpm) during the period in utero. It has been measurable ultrasonographically from about 6 weeks and the normal range varies during gestation, increasing to about 170 bpm at 10 weeks and decreasing thereafter to about 130 bpm at term.

Evolution through gestation
Although the myocardium begins to contract rhythmically within 3 weeks after conception (from spontaneously depolarized myocardial pacemaker cells in the embryonic heart) it is first visible on ultrasound around 6 weeks of gestation. The HRF is therefore usually around 100-120 beats per minute (bpm).

FHR then progressively increases over the next 2-3 weeks becoming:

~ 110 bpm (average) within 5-6 weeks
~ 170 bpm within 9-10 weeks

A decrease in FHR follows which becomes on average:

~ 150 bpm within 14 weeks
~ 140 bpm within 20 weeks
~ 130 bpm per term
Although the heart rate is generally regular in a healthy fetus, a beat-to-beat variation of approximately 5-15 beats per minute can be allowed.

Related pathology
A slow fetal heart rate is termed fetal bradycardia and is usually defined as:

FCF <100 bpm prima della 6,3 settimane di gestazione, oppure
FCF <120 bpm tra 6,3 e 7,0 settimane

A rapid fetal heart rate is called fetal tachycardia and is usually defined as:

FCF >160-180 bpm 5,7
HR around 170 bpm can be classified as borderline fetal tachycardia
A rapid and irregular fetal heart rate is usually referred to as fetal tachyarrhythmia.

How heart rate is listened to and monitored

Your baby's heartbeat can be heard in many different ways, at regular intervals (intermittent auscultation) or continuously (electronic fetal monitoring (EFM).

Intermittent auscultation

It is where your baby's heartbeat is heard at regular intervals with a pinard or a small, portable ultrasound device called the Doptone. If you are in good health and have had a smooth pregnancy, this is the recommended way to listen to your baby's heartbeat during labor. When pregnancy and labor are not complicated, research has now shown that the use of continuous electronic fetal monitoring does not improve the outcome for the baby.
Midwives and doctors listen to your baby's heartbeat for a full minute, every 15 minutes once labor is established, and then more frequently as you approach delivery.

Continuous Electronic Fetal Monitoring (EFM)

It is where your baby's heartbeat is heard continuously using a machine that produces a paper print called a cardiotocograph (CTG). The EFM machine works with two pads (transducers) each the size of a coaster, held in place on the abdomen with two elastic straps. One is positioned towards the upper part of your abdomen, so that it is above the upper end of your womb (uterus) to collect your contractions; the second will be placed on your abdomen, above the area where your baby's heartbeat can best be heard.

The information received from the transducers is converted by the electronics inside the machine to produce a printout on graph paper. EFM using the two external transducers is a non-invasive method. Sometimes, for reasons that will be explained to you, the baby's heartbeat is detected using a small electrode that is placed on the baby's head and connected with a thin wire to the machine, and this would mean that you should have an internal (vaginal) exam in order for that happens.

How to hear it

L'angel sound is a household device that detects the fetal heartbeat, a sort of miniaturization of the detector used during the obstetric visit. There are some with headphones or with loudspeaker and display and just place them on your stomach to hear the baby's heart beating.

Fetal heartbeat detectors are safe and reliable products, as long as they bear the approval mark for import into Europe (the CE mark), which guarantees their quality. And they are safe for the fetus too.

The angel sound exploits thedoppler effect mechanical sound waves, similar to sonar that are used by ships to detect the depth of the seabed. The advice is to use it from the 12-14th week of pregnancy up to the 20th week.

Abnormal fetal heartbeat

The rhythm of a healthy heart requires careful coordination to properly pump blood throughout the body. This rhythm is controlled by electrical impulses that allow for synchronized filling and emptying of the heart's four chambers. Many conditions can cause the electrical impulses that control the heart to be irregular, too fast (tachycardia), or too slow (bradycardia).

Fetal heart arrhythmia, or irregular heartbeat, is a common reason for referral to a fetal cardiologist. In most cases, a heartbeat that is too slow or too fast is temporary. In these cases, your team will closely monitor your pregnancy. Less than 2% of fetal heartbeat irregularities represent true cardiac arrhythmias. However, if cardiac arrhythmia is not diagnosed or treated, it can lead to serious problems for the baby, including congestive heart failure.

By 16 weeks of pregnancy, the fetus's heart is fully formed and beats at a rate of between 110 and 160 beats per minute (bpm). Mild arrhythmia cases were once thought to be benign, but doctors now recognize that any irregular heartbeats should be identified and monitored closely to ensure the best outcome for the baby.

Fetal heart arrhythmias are often classified as one of the following:

Bradycardia - Heart rate below 100 bpm
Congenital heart block
Premature atrial contractions (PACs)
Supraventricular tachycardia or atrial flutter - Heart rate greater than 180 bpm

Symptoms and causes

The condition is often first noticed when the doctor listens to the fetal heartbeat at around 10-12 weeks of pregnancy. However, the rhythm abnormality may not begin before pregnancy. Typically, the mother has no symptoms and does not notice any changes in fetal movement.

The cause of most arrhythmias is unknown, but some cases can result from an electrolyte imbalance, inflammation, medications, or an inherited genetic condition. Severe cases of arrhythmia can be caused by a heart defect such as congenital heart block or by an inherited condition known as long QT syndrome.

Test and diagnosis

If arrhythmia is suspected, additional tests will be ordered, including a fetal echocardiogram. This test will help your fetal cardiologist diagnose if your baby's arrhythmia is caused by a problem with the heart's structure.

Fonti articolo: Radiopedia - Fetal heart rate in the first and second trimester, NHS - Listening to your Baby's Heartbeat, Texas Childrens

Updated on 03.06.2022

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