Fetal heart rate in the first and second trimester
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A normal fetal heart rate (FHR) usually ranges from 120 to 160 beats per minute (bpm) in the in utero period. It is measurable sonographically from around 6 weeks and the normal range varies during gestation, increasing to around 170 bpm at 10 weeks and decreasing from then to around 130 bpm at term.
Evolution through gestation
Although the myocardium begins to contract rhythmically by 3 weeks after conception (from spontaneously depolarizing myocardial pacemaker cells in the embryonic heart) it is first visible on sonography around 6 weeks of gestation. The FHR is then usually around 100 to 120 beats per minute (bpm).
FHR then increases progressively over the subsequent 2-3 weeks becoming 7:
- ~110 bpm (mean) by 5-6 weeks
- ~170 bpm by 9-10 weeks
This is followed by a decrease in FHR becoming on average:
- ~150 bpm by 14 weeks
- ~140 bpm by 20 weeks
- ~130 bpm by term
Although in the healthy fetus the heart rate is usually regular, a beat-to-beat variation of approximately 5 to 15 beats per minute can be allowed.
A slow fetal heart rate is termed fetal bradycardia and is usually defined as 1:
- FHR <100 bpm before 6.3 weeks gestation, or
- FHR <120 bpm between 6.3 and 7.0 weeks
A rapid fetal heart rate is termed a fetal tachycardia and is usually defined as:
- FHR >160-180 bpm 5,7
- FHR around 170 bpm may be classified as borderline fetal tachycardia
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