- ATHENS OFFICE MED4WOMAN ATHENS
- VOLOS OFFICE MED4WOMAN VOLOS
Obstetrician-Gynecologist
PhD, University of Athens Medical School
Obstetrician-gynecologist Panagiotis Polyzos practices at the Institute of Life, the IVF Unit of IASO Maternity Hospital.
Cardiotocography – NST (Non-Stress Test) is a common prenatal test used to assess the health of the fetus. During this examination, accelerations of the fetal heart rate following fetal movements are evaluated (approximately four to six fetal movements are monitored), as well as the variability of the heart rate. NST is also used to record uterine contractions.
With cardiotocography, the fetus is monitored throughout labor, and therefore, you should be aware that a similar machine will be placed next to you during childbirth.
Normally, in healthy fetuses, the heart rate accelerates or increases in response to fetal movements triggered by various stimuli. However, conditions such as fetal hypoxia — when the fetus does not receive adequate oxygen — can disrupt this response.
Your doctor or midwife may recommend cardiotocography after 28–32 weeks of pregnancy if you have:
Your doctor or midwife may also recommend that you undergo NST after 35 weeks of pregnancy, once or twice a week — and occasionally daily — depending on your health and the health of your baby.
Typically, cardiotocography lasts 20–25 minutes. However, if your baby is not active or is sleeping, the NST may need to be extended for an additional 20–25 minutes — with the expectation that your baby will become active — to ensure correct and accurate results.
After the procedure, once the NST is completed, the results are evaluated by your doctor or midwife.
Before 32 weeks of pregnancy, the results are considered normal if your baby’s heart rate accelerates to a certain level above the baseline two or more times, each lasting at least 10 seconds, within a 20-minute recording.
After 32 weeks of pregnancy, the results are considered normal if your baby’s heart rate accelerates to a certain level above the baseline two or more times, each lasting at least 15 seconds, within a 20-minute recording.
Sometimes, during a cardiotocography examination, abnormalities in the fetal heart rate may be detected, requiring further monitoring or treatment.
Possible causes of an abnormal (non-reassuring) result — apart from the fetus possibly being asleep — include reduced oxygen supply to the fetus (fetal hypoxia), maternal smoking, systemic use of certain medications by the mother, as well as underlying neurological or cardiac abnormalities in the fetus.
A biophysical profile combines the NST with a detailed fetal ultrasound that evaluates your baby’s breathing movements, gross body movements, muscle tone, and the amount of amniotic fluid. Additionally, Doppler assessment (PI) is performed to evaluate blood flow in both uterine arteries, the middle cerebral artery, and the umbilical cord.
The test is done in the office or at the maternity hospital and requires no intervention. You lie in a semi-reclining position or tilted slightly onto your left side, so that the uterus does not press on the large veins of the abdomen.
You will hear the baby's heartbeat throughout. If the sound drops out for a few seconds, the baby has usually just changed position and the sensor needs a small adjustment.
Accelerations are the best-known criterion, but full interpretation of a cardiotocograph relies on four features:
None of these is read in isolation. The doctor combines the tracing with the gestational age, your history and the other findings.
The lower curve of the cardiotocograph often causes concern, especially when “waves” appear without you feeling any pain. The tocodynamometer records how often contractions come and how long they last. It does not accurately measure their strength, because the reading depends on how tight the belt is and on the thickness of the abdominal wall.
Braxton Hicks contractions are very common in the third trimester: irregular, usually painless, and easing with rest or a change of position. Labor begins when contractions become regular, closer together and stronger, and change the cervix.
The NST does not show whether the cervix is opening. If preterm labor is suspected, a vaginal exam or measurement of cervical length by transvaginal ultrasound is needed. Likewise, a tracing without contractions does not rule out labor starting in the next few days.
During labor, fetal monitoring is not done the same way for every woman. For healthy women with an uncomplicated singleton pregnancy and no risk factors, NICE guidelines recommend intermittent auscultation of the heart rate at regular intervals. Continuous cardiotocography is preferred when there are reasons for closer surveillance, for example:
The tracing is usually classified into three categories, normal, suspicious or pathological, based on FIGO or NICE criteria. A suspicious tracing leads to simple corrective measures, such as a change of position, hydration or reducing oxytocin. If the signal from the abdomen is poor and the membranes have ruptured, a small electrode may be placed on the baby's scalp for a more reliable recording.
A nonreactive result does not automatically mean there is a problem, nor does it mean immediate delivery. A logical sequence of steps follows:
The decision on the next step always depends on gestational age. At 39 weeks the doctor will think differently than at 31.
A two-channel machine is used so that each baby's heart rate is recorded simultaneously and separately. The doctor confirms with ultrasound that the two sensors are “hearing” different babies and not the same one twice.
Before week 32 the nervous system is less mature. Accelerations are smaller and variability is often lower, which is why more relaxed criteria apply.
The signal may drop out more often and the recording may have gaps. A change of position or using ultrasound for confirmation often helps.
In every case there is one risk that is easy to check: the machine accidentally recording the mother's pulse. That is why the doctor or midwife compares the reading with your own pulse.
Scheduled NSTs cover pregnancies with a known risk factor. However, some symptoms call for a same-day recording, no matter when your next appointment is:
In women with gestational diabetes that requires insulin, regular monitoring in the final weeks of pregnancy is usually part of the follow-up plan. If you are not sure whether a symptom is serious, contact your gynecologist. A recording that turns out to be unnecessary harms no one.
Usually from week 36 to 37 in low-risk pregnancies, and earlier when there is an indication.
About 20 to 40 minutes.
That the expected heart rate accelerations did not appear during the recording. Often the fetus is simply asleep. The test is extended or supplemented with a biophysical profile.
No. A light meal beforehand helps, because the baby is more active.
Yes, it is completely harmless for the mother and the fetus.
The sensor targets a small spot. When the baby changes position or you move, the heartbeat may drop out for a few seconds. This is not a cause for concern. The midwife simply repositions the sensor and the recording continues.
Not necessarily. Irregular Braxton Hicks contractions are common in the third trimester. Labor begins when contractions become regular and painful and change the cervix, which is assessed by examination and not from the curve.
Yes. Keep it with your other pregnancy test results. If you need to visit the maternity hospital, comparing it with a previous recording helps the doctors assess the situation faster.
No. The device only picks up the heartbeat for a few seconds and does not show variability, accelerations or decelerations. It can also give false reassurance if it picks up your own pulse. Reduced movements always require a proper recording.
