REF-1401 · Wowdzia et al., 2023 · Sports Medicine · Paper · PubMed

Are hard 1-minute intervals OK while pregnant?

In 15 women at ~27 weeks, 10 × 1 min peaking near 96% HRmax caused no fetal bradycardia before or right after, as with 30 min moderate. One session each; not measured mid-rep.

Main caveat: Read from the abstract only. Exercise mode, training status, inclusion criteria, the timing of fetal measures and adverse events could not be checked.
limited evidenceMeasured on non-athletes to trained athletes

Wowdzia and colleagues compared the maternal and fetal cardiovascular responses to one session of high-intensity interval training (HIIT) and one of moderate-intensity continuous training (MICT) during pregnancy.

Fifteen women with a singleton pregnancy (27.3 ± 3.5 weeks of gestation, 33 ± 4 years) did a peak fitness test and then, 48 h apart in random order, HIIT (10 × 1 min at 90% or more of maximum heart rate with 1 min of active recovery) and MICT (30 min at 64-76% of maximum heart rate).

Maternal heart rate, blood pressure, middle and posterior cerebral artery blood velocity and respiratory measures were recorded throughout; fetal heart rate and the umbilical artery systolic/diastolic ratio, resistive index and pulsatility index were assessed immediately before and after exercise.

Average maternal heart rate was higher in HIIT (82 ± 5% of maximum) than in MICT (74 ± 4%; p < 0.001), and HIIT peaks reached 96 ± 5% (range 87-105%). Maternal cerebral blood velocities rose with exercise and did not differ between sessions.

Fetal heart rate rose by 14 ± 7 bpm with HIIT and 10 ± 10 bpm with MICT, with no difference between sessions; umbilical flow indices fell with exercise and did not differ between sessions. No fetal bradycardia was observed, and the umbilical indices stayed within normal ranges before and immediately after all sessions.

The authors conclude that an acute bout of HIIT with repeated 1-min near-maximal to maximal efforts, like MICT, is well tolerated by mother and fetus. Our reading: this is the counterweight to the 90% line from REF-1400: brief reps that peaked near 96% of maximum did not produce bradycardia detectable before or straight after the session in 15 women.

It does not overturn that observation, which came from treadmill workloads in Olympic athletes measured during exercise (stage durations not in that abstract); together they say that no heart-rate number has been tested as a limit.

Protocol

Design: randomised crossover trial (NCT05369247); peak fitness test, then HIIT and MICT sessions 48 h apart in random order.
Who: n = 15 women with a singleton pregnancy, 27.3 ± 3.5 weeks of gestation, 33 ± 4 years; inclusion criteria, training status and pregnancy risk status are not in the abstract.
HIIT: 10 × 1-min intervals at ≥90% of maximum heart rate, with 1 min of active recovery between them.
MICT: 30 min at 64-76% of maximum heart rate.
Maternal measures (continuous): heart rate, blood pressure, middle and posterior cerebral artery blood velocity (MCAv, PCAv), respiratory measures.
Fetal measures (immediately before and after exercise): fetal heart rate; umbilical artery systolic/diastolic (S/D) ratio, resistive index (RI) and pulsatility index (PI).
Not in the abstract: exercise mode, how maximum heart rate was established (the abstract does not say), the timing of the post-exercise fetal measurement, adverse events, and confidence intervals.
Funding and interests: Crossref lists the Heart and Stroke Foundation of Canada, the Natural Sciences and Engineering Research Council, the Christenson Professorship in Healthy Active Living and a National and Alberta Heart and Stroke Foundation of Canada New Investigator Award as funders; PubMed lists Natural Sciences and Engineering Research Council grant RGPIN-07219.A competing-interests statement is not available from the abstract or the PubMed record. No device maker, federation or data platform is named. Davenport is also an author of the 2019 Canadian pregnancy guideline (REF-568) and of REF-1395; an author overlap, not a declared conflict.

Key Results

MeasureHIIT (10 × 1 min)MICT (30 min)Between sessions
Average maternal heart rate, % of maximum82 ± 5%74 ± 4%p < 0.001
Peak maternal heart rate, % of maximum96 ± 5% (range 87-105%)——
Middle / posterior cerebral artery velocityrose with exerciserose with exercisep = 0.340 / p = 0.142 (no difference)
Fetal heart rate change, before to after+14 ± 7 bpm+10 ± 10 bpmnot different
Umbilical PI / S/D ratio / RIfell with exercisefell with exercisep = 0.707 / 0.671 / 0.792 (no difference)
Fetal bradycardianonenone—
Umbilical indices before and immediately afterwithin normal rangeswithin normal ranges—
Reading the nulls: the between-session comparisons are p-values without confidence intervals in 15 women, so they are no evidence of a difference, of unknown precision; a modest difference in fetal heart rate or umbilical indices cannot be excluded from the abstract.
A wording that does not add up: the abstract says fetal heart rate "increased during exercise (p = 0.244)", although the methods measured it before and after, and p = 0.244 is not a significant change; which comparison that p-value belongs to is not stated.
The peak range: individual peaks ran from 87% (at least one woman never reached the 90% target) to 105% of maximum (at least one exceeded the maximum from her peak test, so that test did not capture her true maximum; our inference).

Mechanisms

Measured: HIIT raised average and peak maternal heart rate above MICT; cerebral blood velocity rose similarly in both; fetal heart rate rose and umbilical flow indices fell after both, with no difference between them and no bradycardia.
Authors' interpretation: repeated 1-min near-maximal to maximal efforts are well tolerated by mother and fetus.
Inference: a 1-min rep with 1 min of recovery holds the mother near maximum only briefly, and average heart rate over the session was 82% of maximum; in REF-1400 bradycardia appeared during treadmill workloads above 90% (their durations are not in that abstract) when uterine blood flow fell below half its initial value.The two studies may therefore describe different exposures (brief peaks vs continuous treadmill stages, if those stages were long) rather than contradict each other.Because fetal heart rate was measured before and after rather than during the reps, a transient slowing during a rep that recovered within the following minutes would not be seen here; the abstract gives no timing for the post-exercise measurement.

Practical Application

Are hard 1-minute intervals OK while pregnant? In 15 women at ~27 weeks, 10 × 1 min peaking near 96% HRmax caused no fetal bradycardia before or right after, as with 30 min moderate. One session each; not measured mid-rep.
Short near-maximal reps were well tolerated in healthy pregnancy (B, randomised crossover, n = 15, one session each, from the abstract). This is the best-controlled data in the base on brief efforts above 90% of maximum heart rate in pregnancy, and it argues against treating 90% as a hard ceiling (REF-1400, the study the line is usually traced to: six Olympic athletes, treadmill workloads, one session each).
What it did not show: that sustained efforts near maximum (a 20-min threshold block, a time trial, a race) are safe; that weeks of such training are; anything about elite training volumes, early or late pregnancy, complicated pregnancies or outcomes at birth; and what happens to the fetal heart rate during a rep.
Use the session shape that was tested: ten 1-min reps with 1 min of active recovery (10 min of work in about 20 min; the 30-min session was the moderate one), average 82% of maximum heart rate. Not from this paper; the base's own practice: perceived effort and the talk test govern in pregnancy, longer near-maximal efforts are eased late in pregnancy, and the obstetric team sets any limit; contraindications and warning signs come first (REF-567).
What it is good for as a source: a randomised, within-woman comparison of brief maximal reps and moderate continuous exercise on fetal heart rate and umbilical flow. What it does not show: a safe heart rate; chronic effects; responses in athletes training at high volume.

Limitations

Read from the abstract only. Exercise mode, training status, inclusion criteria, the timing of fetal measures and adverse events could not be checked.
Fetal measures before and after, not during. A transient bradycardia during a rep that resolved before the post-exercise measurement would be missed.
Acute, small and single-session. 15 women, one session of each kind; no chronic training, no birth outcomes.
Maximum heart rate from a peak test. Peaks up to 105% suggest that test underestimated some women's maximum, so "≥90%" is relative to a test value, not a true maximum (our inference).
Funding and interests: public research funders (Heart and Stroke Foundation of Canada; Natural Sciences and Engineering Research Council; a Christenson Professorship) per Crossref and PubMed; no competing-interests statement available.
How this entry was checked and changed
Why this level

(from the abstract) — a randomised crossover with 15 participants meets the rubric's bar for B+; held one step lower because the fetal measures were taken immediately before and after each session, not during the intervals (in REF-1400 bradycardia appeared during exercise and normalised quickly after stopping), the outcomes are acute fetal markers after one session of each kind, between-session comparisons are given as p-values without confidence intervals, and the abstract gives neither the exercise mode nor the women's training status nor how maximum heart rate was set. Read from the abstract (closed access; Europe PMC holds no full text and OpenAlex lists no open copy)