Yes, fetal heart rate does decrease during the second trimester, though the change is gradual rather than dramatic. After peaking early in pregnancy (typically around 170 bpm near weeks 9 to 10), the heart rate settles into a range of roughly 139 to 151 bpm during the second trimester and continues declining into the third trimester, where it averages 119 to 131 bpm. This steady downward trend is a normal sign of your baby’s maturing nervous system.
What the Numbers Look Like Week by Week
During the first trimester, fetal heart rate climbs rapidly from the first detectable flicker around week 6 to a peak that can reach the upper 160s. By the time you enter the second trimester at week 13, the rate has already begun easing down. Studies using both ultrasound and fetal electrocardiography find an average heart rate of about 148 to 149 bpm at 20 to 24 weeks, which sits comfortably in the normal second trimester range of 139 to 151 bpm.
So if you notice that your baby’s heart rate was 165 at your 12-week appointment and drops to 145 at your 20-week anatomy scan, that’s completely expected. The decline continues through the rest of pregnancy, with third trimester rates typically falling between 119 and 131 bpm at rest.
Why the Heart Rate Slows Down
Early in pregnancy, the fetal heart is primarily driven by the sympathetic nervous system, the same “gas pedal” system that speeds up your own heart when you’re startled. As the second trimester progresses, the parasympathetic nervous system (the “brake pedal”) begins to mature and exert more influence. The vagus nerve, which runs from the brain to the heart, gradually gains the ability to slow the heartbeat.
This shift in control is also moving upward in the brain itself. Early on, basic heart rate regulation happens in the brainstem. During the late second and into the third trimester, higher brain regions start participating in autonomic control. The result is a slower resting heart rate paired with greater heart rate variability, meaning the baby’s heart becomes better at speeding up and slowing down in response to movement, sleep, and stimulation. Both of these changes are markers of healthy neurological development.
Sleep-Wake Cycles Add Fluctuation
On top of the overall downward trend, the fetal heart rate isn’t constant throughout the day. Research using 24-hour monitoring has found that fetuses as young as 16 to 20 weeks already show daily rhythms in both heart rate and movement. Fetuses tend to have “active” periods (roughly 9 a.m. to 2 p.m. and 7 p.m. to 4 a.m.) and “quiet” periods (4 a.m. to 9 a.m. and 2 p.m. to 7 p.m.).
This means the heart rate your provider captures at one appointment could be several beats higher or lower than the rate a few hours later, simply because the baby was awake versus asleep. A reading of 155 at one visit and 140 at the next doesn’t necessarily reflect a trend. It could reflect the baby’s activity state at the moment the Doppler was placed.
What Counts as Too Low
A sustained fetal heart rate below 110 bpm lasting more than 10 minutes is the standard definition of fetal bradycardia. That threshold applies across the second and third trimesters. Rates below 100 bpm are rare, occurring in roughly 5% of fetuses with arrhythmias.
On the high end, rates consistently above 160 to 170 bpm during the second trimester may warrant closer monitoring. Keep in mind that brief accelerations well above the baseline, especially when the baby is moving, are normal and actually reassuring. It’s the sustained baseline that matters, not momentary spikes.
What You Might Notice at Appointments
Most people first hear the fetal heart rate via Doppler around weeks 10 to 12, when rates are still relatively high. By the 20-week anatomy scan, the rate has typically settled into the mid-to-upper 140s. If your provider has been noting the heart rate at each visit, you may see a clear downward slope across your chart, dropping perhaps 5 to 15 bpm between your early second trimester visit and your late second trimester appointments.
Providers generally note the heart rate as part of routine checks but won’t flag the gradual decline because it’s expected. If you’re tracking the numbers yourself and wondering whether a drop from 160 to 140 is cause for concern, it almost certainly isn’t. That pattern mirrors exactly what a healthy, maturing nervous system produces. The time to ask questions is if the rate falls below 110 at any point or if your provider seems to linger on the reading longer than usual.