But what if ‘normal’ is beyond the average 150 mins or 30 mins 5 x a week? - by far
Many of my clients who are regular runners & cyclists will do that in a weekend - in one session! And what about intensity? ‘Moderate’ is the term often used in advice on pregnancy exercise, but what is that & can you go higher?
I’ve been looking at the most recent research I could find into levels of exercise, particularly the effects on mother & baby in ‘strenuous’ running & cycling. Thankfully NONE have found that ‘vigorous exercise’, categorised as < 60-84% Max HR, even up to (but not sustained) VO2 max, causes stress to the foetus, small birth weights, or early deliveries!(1, 4, 5, 6, 8) . A meta-analysis of relevant research articles found one that even suggested that vigorous exercise in the first & second trimesters may result in healthier placentas by increasing vascularisation (8)
There is however one study that I found, & is mentioned in other research reviews of elite athletes that found regular high intensity workouts (> 90% MHR, 3< x per week) in late pregnancy had lower birth weight (2). However a review of the available evidence with elite athletes found ‘little to no evidence of harm’ (7).
Another found that in very small samples found exercise of >95% max HR resulted in very short periods of foetal reduced HR immediately post exercise (2-10mins) in ‘Highly Active’ women. Importantly there were no lasting effects on foetal wellbeing & the women were categorised according to their pre-pregnancy levels of activity. In this category of women & the elite, it is recommended that further research is needed as they are likely to push to the intensities where foetal wellbeing may be compromised but as yet is not known.(1)
Using maternal HR as a measure for exercise limit is also difficult as there is not a consensus in the literature as to whether HR threshold is altered in pregnancy. In a study of Cardio respiratory response in sub maximal exercise, 70% max HR, pregnant women in their first trimester reached HR & respiratory rate targets sooner, & took marginally longer
to recover . “This suggests that during pregnancy, either the aerobic system is used less efficiently or the aerobic system is already used at higher levels during rest, which fits the hypothesis that pregnancy can be considered as a 24-hr/9-month lasting mild to moderate exercise.” (3)
My conclusion from looking through the available evidence is that in a normal pregnancy of a ‘highly active’ mother it is OK to keep exercising, even up to higher HRs for intervals, but best not >90% HR efforts multiple times a week just because the evidence is not there to rule out complications. Elite women should seek professional guidance & continuous monitoring when training throughout their pregnancies.
In fact it is important to keep up a certain level of exercise. Melzer et al stated that
‘A substantial proportion of women stop exercising after they discover they are pregnant… The adoption or continuation of a sedentary lifestyle may contribute to the development of certain disorders such as hypertension, maternal & childhood obesity, gestational diabetes, dyspnoea, and pre-eclampsia.’(10)
If you stop exercising you will become less fit! Your body will be going through lots of changes for which exercise can reduce the incidence of pregnancy related musculoskeletal pains, so you do not want to decondition- just consider it as ‘reconditioning'
The Canadian guidelines advising obstetric care state that ‘reasonable goals of aerobic conditioning in pregnancy should be to maintain a good level of fitness throughout the pregnancy without trying to reach peak fitness or train for athletic competition’ (9).
There may be other factors that do mean you need to tone down the cardio, such as just feeling knackered! - but you can still tone up your muscles by switching to other forms of training- such as resistance & core work.
Also many active people use exercise for stress relief & socialising. Combine that with an altered sense of ‘purpose’ with someone used to having competition & targets to train for. When that’s taken away their motivation to exercise can be severely depleted. Sometimes a certain level of exercise is needed to get the endorphins & feel good in body & mind. (Definitely another topic to explore further!)
1 Strenuous exercise is pregnancy: is there a limit?
Syzmanski and Satin
Am J Obstet Gynecol. 2012 September ; 207(3): 179.e1–179.e6. doi:10.1016/j.ajog.2012.07.021.
2. Fetal wellbeing may be compromised during strenuous exercise among pregnant elite athletes.
Salvesen KÃ, Hem E, Sundgot-Borgen J.
Br J Sports Med. 2012;46(4):279.
3. The physiological effect of early pregnancy on a woman’s response to a submaximal cardiopulmonary exercise test
Bijl et al.
Physiological Reports. 2020;8:e14624.
4. Physical exercise during pregnancy:a systematic review
Nascimento, Fernanda, Cecatti
Curr Opin Obstet Gynecol. 2012 Dec;2(6):387-94
5. Maternal and Fetal cardiovascular responses to strenuous bicycle exercise
Van Doorn et al.
Am J. Obstet Gynecol 1992 Mar;166 (3) 854-859
6. Sports and Pregnancy
Kagan and Kuhn
https://pubmed.ncbi.nlm.nih.gov/15241543/
7. The Elite Athlete and Strenuous exercise in pregnancy
Pivarnik,Szymanski and Conway
Clin Obstet Gynecol. 2016 Sep;59(3):613-9
8. The effects of vigorous intensity exercise in the third trimester of pregnancy: a systematic review and meta-analysis
Beetham et al
BMC Pregnancy and Childbirth (2019) 19:281
Davies GAL, Wolfe LA, Mottola MF, MacKinnon C.
J Obstet Gynaecol Can. 2018 Feb;40(2):e58-e65.
10. Physical activity and pregnancy:cardiovascular adaptations, recommendations and pregnancy outcomes
Melzer et al
Sports Med. 2010 Jun 1;40(6):493-507
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