Why you should (or shouldn’t) exercise during pregnancy - and what Science says about it

The number one reason that women stop exercising during pregnancy is they feel like it could be harmful to both baby and mom. They say there could be a lack of blood flow and oxygen supply to the baby, ligaments become lax and too unstable to train, it can effect the amount of nutrition to the baby, body temperature is elevated to an unsafe level, or there is too much mechanical stress to sustain. This has been a long held belief by many women, and it’s pretty similar to the myth that strength training for youth athletes is harmful too. At 1RM, we have written articles based on science and practical application that youth strength training is perfectly safe and actually beneficial for athletes. In this post, we will show that science also supports training during pregnancy.

What is actually happening to your body during pregnancy? Obviously, a small living human is being created. But what really goes on and why do we feel tired or nauseous or out of breath? This may be hard to believe, but the changes induced by regular VIGOROUS exercise are actually very similar to those induced by pregnancy. Regular vigorous exercise is associated with an increased blood volume, increased size of heart chambers, increased volume of blood pumped with each beat, maximum cardiac output, and increased density of blood vessels in skeletal muscle. It also improves a persons ability to sweat and lower skin temp required to produce an increase in skin blood flow. These changes are the same as what happens during pregnancy. So much so, that a normal pregnant lady at rest is similar to a trained non pregnant lady during exercise. Training during pregnancy simply superimposes those adaptations on the normal ones. Now that we are starting to understand how training could actually complement pregnancy, let’s look at the 6 major physiological affects of pregnancy. 

#1 Cardiovascular changes:

If you are one of those people who track their heart rate, then you know that shortly into pregnancy your resting heart rate increases significantly (10 beats or so). That’s because hormonal signals from the embryo, fetus, and placenta signal changes in the circulatory system in order to support the increasing needs of the developing child. In fact, this happens just after fertilization. The outer rim of cells that become the placenta release hormonal signals to initiate relaxation and reduced responsiveness of blood vessels in the muscle cells. The elasticity and volume of the entire circulatory system (heart, arteries, and veins) increases virtually overnight. This creates a problem because blood volume hasn’t increased at this point. The amount of blood returning to the heart decreases, as does the amount of blood in the heart pumped out. This means lower blood pressure and an under-filled vascular system. The symptoms? Light-headed-ness, nausea, fatigue, cravings, constipation, bloating, frequent urination, sweating, racing pulse, dizziness and so on. 

In response to an under-filled system, hormones are released from the heart and adrenal gland, which cause the body to decrease the excretion of salt and water by the kidneys. This extra salt and water correct the under-fill problem by expanding the volume of the plasma. The heart volume also increases (amount of blood pumped with each beat) by 15-20%. 

Normal non-pregnant lady : High resistance, average volume, normal flow rate circulatory 

Pregnant lady : low resistance, high volume, high flow (needed for growth of the baby)

The vascular relaxation and dilation of the blood vessels are most pronounced in the vessels that supply the skin, kidneys and reproductive tissues. In fact, it could increase by 2-20x the normal rate! The result:

  • Flushed and warm especially in palms and face (increase in skin blood flow increase skin temperature)
  • Increased urination (increases in kidney blood flow improves waste removal and increased volume of urine)
  • Uncomfortable sensation of pelvic and lower abdominal fullness (increased blood flow to reproductive tissue ensures adequate delivery of oxygen/nutrients)

#2 Circulatory changes:

During pregnancy, there is a 10-15% increase in plasma volume, red cell volume, and total blood volume in exercising women. Women who continue to train have more circulatory reserve. This improves their ability to deal with anticipated work (exercise/work) and unanticipated (hemorrhage, trauma, anesthesia, so on).

This is key. Many women think exercise decreases blood flow to the placenta and baby, potentially putting the baby at risk. It is true, during exercise, blood supply is increased to the muscles, skin, and adrenal glands and is decreased to renal, GI, and reproductive systems. But, the placenta is structurally designed to maximize oxygen delivery to the baby even during times of decreased oxygen. Research has shown that these times of lower oxygen flow can actually help the baby deal with unanticipated stress (labor, car accident, hemorrhage, etc). 

#3 Pulmonary changes: 

During pregnancy, the amount of air consumed increases by 40-50%. This is the result of elevated levels of progesterone, initiating over breathing. This increases sensitivity of the respiratory center in the brain to carbon dioxide, and improves the transfer to gases to and from the baby. The symptoms? Breathlessness at rest and during mild exercise.

The placenta actually plays a big role here and helps with the efficiency of oxygen and CO2 transfer between baby and mother. The placenta’s of women who exercise early and mid pregnancy have been shown to grow faster and function better. This means more oxygen and nutrients delivered to the baby. If you studied Exercise Science in college, or took any exercise physiology class, you probably had to test your VO2max. This is the maximal amount of oxygen that can be consumed during exercise. Basically you run or bike until you physically can’t go any further. The crazy thing about pregnancy is your VO2max actually increases by 5-10%! And if you keep training through your whole pregnancy, those gains are still apparent 6-12 months  post baby. National and international track athletes post-baby have improved their sprint times because pregnancy is similar to blood doping. 

Both pregnancy and exercise improve the ability of body tissues to take up and utilize oxygen. Exercise during pregnancy:

  • increases alveolar ventilation
  • improves gas transfer at tissue level
  • maintains peak ventilation
  • maintains max aerobic capacity
  • increases blood volume 
  • makes it easier to maintain higher blood flow rate to baby
  • enhances placental gas transfer

#4 Body temperature:

Early in pregnancy, the increase in progesterone increases body temperature, but body tempeature falls mid to late pregnancy. This means that getting rid of excess heat is not a problem for several reasons. For one, there is a 40-50% increase in ventilation which helps to expel heat. Think of a dog panting on a hot day to cool himself, pregnant ladies just expel the heat through breathing. The second reason is that the the amount of tissue or surface area of the skin increases by 5-10% early to mid pregnancy, and 20-25% by the end of pregnancy. All that extra surface area is valuable in expelling heat. The bodies set point for sweating is also decreased, meaning sweat comes quicker during exercise and helps to keep pregnant ladies cool. With all these changes to a woman’s body temperature, it would be natural to assume that the babies temperature is going haywire. But research has shown that even during strenuous exercise a babies temp doesn’t change. In fact, exercising pregnant ladies can deal more effectively with heat and stress, meaning baby stays nice and cool and safe. 

#5 Metabolic changes:

Resting metabolic rate increases by 15-20% during pregnancy. If you read any pregnancy book, most recommend a healthy amount of weight to gain is 25-35 pounds. But where does this weight come from? Maternal fat stores account for about 7-11 pounds, which is 21,000 to 35,000 calories of fuel. These extra calories are important for baby and for mom, because later in pregnancy women rely more on fat as a fuel source and save carbohydrates as energy for fetal growth. Fetoplacental growth accounts for another 7-11 pounds. The last weight gain is due to fluid retention and blood volume, tallying another 9-15 pounds. 

#6 Musculoskeletal changes:

Unfortunately, there isn’t much research done on muscle, bone and ligament adaptations in women during pregnancy. An enlarging belly puts more mechanical stress on the back, pelvis, hips and legs. At the same time, the ligaments around the pelvis and back start experiencing some loosening and stretching. Doctors are concerned with this instability because ankles, knees or hips could be more prone to injury. However, the limited research done in this area has shown the only usual complaint from women is low back pain. It also has showed bone health is improved because estrogen and progesterone help with bone turnover, remodeling and density. Some studies indicate that pregnancy may actually increase muscle mass by 5% because of carrying an extra 20 or so pounds daily, but that research is limited. 

The physiological changes occuring during pregnancy are immense. But, research has shown that the effects of frequent (5 or more times per week), prolonged (30-90 minutes per session), high-intensity weight-bearing exercise (65-90% of maximal capacity) could complement the changes of pregnancy. Women who maintain frequent, prolonged, high intensity exercise throughout the whole pregnancy have a 10% increase in maximal aerobic capacity postpartum; but this only occurs when they exercise through the entire pregnancy. They also experience about 7 pounds less weight gain and decreased fat retention. Furthermore, studies have shown that 3 months post-baby, the weight loss difference between the exercising and non-exercising group was only 2 pounds different. BUT, the biggest differences were found 6-12 months post-baby. Women lost weight much easier during this time frame, and actually became leaner and lighter than before pregnancy. 

Stay tuned for the next article in this series on training through your 3rd trimester!

Exercising through your Pregnancy. Catherine Cram and James F. Clapp