Today, healthy women are encouraged to engage in daily physical activity throughout pregnancy.1–3 Both endurance training and strength training are recommended, and from a health perspective pregnant women are encouraged to engage in 30min of moderate intensity aerobic training every day.1 Davies et al.2 recommend strength training of the major muscle groups 3–4 times per week and suggest that abdominal strength training is important to strengthen “the muscles of labor”.

Several studies have investigated the level of physical activity4–6 and exercise training7 during pregnancy in population-based studies. However, to date, there is scant knowledge to which extent pregnant women perform abdominal exercises. Strong abdominal muscles have been claimed to contribute to a more effective birth in terms of shorter duration of second stage of labor.8–10 Furthermore, Bovbjerg and Siega-Riz11 have postulated that strong abdominal muscles might make the second stage of birth more effective, thereby reducing the risk of failure to progress and cesarean section. The theory is that when the women are asked to actively push during the uterine contractions, strong and well-trained abdominals would improve the effectiveness of the pushing and thereby shorten the duration of the second stage of labor. Despite the Canadian recommendations2 and the aforementioned theories,8–11 there is a paucity of research addressing a possible association between strength training of the abdominal muscles and delivery mode.12 For this reason, the Norwegian Mother and Child Cohort Study (MoBa) included questions on abdominal training. MoBa is linked to the Medical Birth Registry of Norway (MBRN) and therefore allows analysis of exercise exposure and birth outcome.

The aims of the present study were to investigate:

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  The number of women reporting to engage in strength training of the abdominal muscles before and during pregnancy.

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  The association between self-reported abdominal strength training before and during pregnancy and acute cesarean section, instrumental assisted vaginal delivery and third- and fourth-degree perineal tear.

Background variables are shown in Tables 1 and 2. The majority of the women had normal pre-pregnancy BMI, had completed higher education (college/university), and was married or cohabitants.

Amongst participants, 3999 (11.1%) underwent acute cesarean section, 6382 (17.7%) instrumental assisted vaginal delivery (forceps and vacuum), and 2051 (5.7%) had third- or fourth-degree perineal tear.

Numbers and percentages of women reporting to perform abdominal strength training before and during pregnancy are reported in Table 3. During pregnancy, there was a significant decline in number of women reporting abdominal strength training (p<0.001). Forty-seven percent of the women reduced their frequency of abdominal strength training from 3 months pre-pregnancy to gestational week 17. There was a further reduction in frequency of abdominal strength training from gestational week 17 to gestational week 30, 27% of the women reported to reduce their activity. At gestational week 30, 31% of the women reported to do abdominal strength training.

Table 4 shows crude and adjusted odds ratios for report of abdominal strength training 3 months before pregnancy and acute cesarean section, instrumental assisted vaginal delivery, and third- and fourth-degree perineal tear. There was no significant association between abdominal strength training and any of the delivery outcomes. Adjusting for fear of childbirth, dystocia, induction of labor, epidural, episiotomy (for perineal tear only), or instrumental assisted vaginal delivery (for perineal tear only) had no influence on the results.

Table 5 shows the crude and adjusted odds ratios for report of abdominal strength training before pregnancy and at all time points during pregnancy combined and the delivery outcomes. There was no association between either varied training frequency or training with the same frequency before and during pregnancy with mode of delivery or perineal tears. Adjusting for plausible confounders (fear of childbirth, dystocia, induction or epidural, episiotomy (for perineal tear only), instrumental assisted vaginal delivery (for perineal tear only)) had no influence on the results.

The main findings of this prospective pregnancy cohort study on abdominal strength training and delivery outcome were that two-thirds of the women reported to engage in strength training of the abdominal muscles before pregnancy. This declined to a third of the participants at gestational week 30. However, there was no association between maternal reports of abdominal strength training before and during pregnancy and acute cesarean section, instrumental assisted vaginal delivery, or third- and fourth-degree perineal tears.

The main strengths of the study are the large sample size and the access to longitudinal data on several exposures and plausible confounders. The MBRN is considered a reliable source of information related to birth,17 and in Norway this registration is mandatory for all women giving birth. The follow-up rate of more than 90% also strengthens the study.13 The study's hypotheses was not known to the women when they answered the questionnaires, which may limit the potential impact of information bias.

The main limitation of the study is the use of questionnaire data to assess frequency of abdominal strength training without any clinical assessment of actual abdominal muscle strength. Self-report may overestimate all training estimates and recall bias is a possible threat to the accuracy of self-report, in general. In the present study, retrospective maternal report of training 3 months before conception may be a special weakness.18 In addition, we have no information of the type of abdominal exercises (e.g. sit up or core stability training). Even with reliable self-reporting, there is no guarantee that the conducted abdominal strength training resulted in stronger abdominal muscles. Nevertheless, to date, there is scant knowledge about the effect of abdominal strength training in general during pregnancy, and as far as we have ascertained there are no studies evaluating the validity of report of abdominal strength training and actual increase in muscle strength. Combining the answers from three exposure points into one variable may improve the validity of the report as it indicates that the responders are “true” exercisers. Low response rate is one of the main challenges of conducting population-based studies. Nilsen et al.14 evaluated the differences between the participants in MoBa and the population in general to see whether there was a case of selection bias in MoBa. They found that younger and single women were underrepresented in MoBa, as also smokers. There were also a lower rate of preterm deliveries, lower gestational age, and babies with higher Apgar score and larger head circumference in the MoBa group. This can indicate a socioeconomic difference between MoBa participants and the population, in general.13 Such differences might affect the associations between the exposures during pregnancy and different outcomes.14 Thus, we cannot exclude that selection bias might have influenced our results. The gold standard design to rule out causality for abdominal strength training to influence delivery outcome would be a randomized controlled trial (RCT). However, given the low incidence of the main outcomes, a randomized controlled trial with these as main outcome variables would require a huge sample size and may not be feasible.

A few RCTs have reported the effect of strength training in relation to pregnancy and delivery.19–23 None of these studies found differences between the group that performed strength training and the group that did not train on delivery outcomes (acute cesarean section, instrumental assisted vaginal delivery). However, none of these RCTs had a primary aim to investigate the effect of abdominal strength training alone on acute cesarean section rate, instrumental assisted vaginal delivery, and third- and fourth-degree perineal tear. In addition, none measured abdominal strength before and after the intervention and none reported on which abdominal exercises that had been performed. Hence, to date, the evidence for the effect of abdominal strength training on delivery outcome is not clear.

To date, there is also scant knowledge about normal activity of the abdominal muscles during pregnancy and labor. Early studies from the 1950s and 1960s found that the electrical activity of the abdominal muscles decline as the pregnancy progresses.24,25 More recently, Oliveira et al.26 confirmed that there is activity in the abdominals during labor. They also found a negative correlation between the diastasis recti abdominis and electrical activity in m. rectus abdominis, but no correlation between the activity in m. rectus abdominis and m. obliquus externus and duration of second stage of labor. Buhimschi et al.27 investigated the change in intra-uterine pressure during contractions in the second stage of labor and found an increase of 62% when the mother performed the Valsava maneuver. To the best of our knowledge, there are no published studies on the effect of strength training on the abdominal muscles and the ability to increase intra-abdominal pressure in pregnant women. There is also uncertainty to the effect of different pushing techniques during delivery (open or closed glottal slit) and delivery mode.28,29 Women's health physical therapists are in close contact with pregnant women and are often asked questions about exercise during pregnancy. It is important that the advices and recommendations given by health personnel are evidence-based. Current recommendations for abdominal strength training during pregnancy are limited to advice against doing exercises in the supine position after the fourth month of pregnancy.30 To date, there is sparse knowledge on which abdominal exercises are safe for pregnant women both before and during pregnancy and especially the effect of abdominal training on birth outcome. The results of the present study indicate that abdominal training may not influence birth outcomes. However, there is an urgent need for further clinical studies to elaborate on this issue, both the role of the abdominal muscles during delivery and the effect of abdominal training during pregnancy on abdominal strength and how it may affect other outcomes. Hopefully our results will stimulate to more research.

A third of the participating women engaged in strength training of the abdominal muscles before and during all time points of their pregnancy. However, there was no association between self-reported abdominal strength training and delivery outcomes in this large population-based pregnancy cohort study. To be able to give pregnant women advice regarding abdominal strength training there is an urgent need for further research.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sector. The Norwegian Mother and Child Cohort Study is however supported by the Norwegian Ministry of Health and Care Services and the Ministry of Education and Research, NIH/NINDS (grant no.1 UO1 NS 047537-01 and grant no. 2 UO1 NS 047537-06A1).