Would blood lactate from exercise be of harm to the fetus of my pregnant client (she is 23 years old and untrained). A scientific reply would help greatly.
In the absence of specific client information, before I give you a scientific response to your question I would like to focus on one word which I feel may be extremely relevant in this instance: “untrained.” The ACSM’s current comment states that pregnancy is not a time for aiming to improve fitness levels, and therefore any exercise undertaken should not exceed pre-pregnancy levels. This implies that as an untrained participant, it is not advisable to allow your client to work in a zone that may build significant lactic acid levels in her blood.
Looking specifically at the research that has been done regarding blood lactate and its effects on the fetus, there must be one major consideration: how would this be measured? Monitoring the fetal heart rate, its accelerations and decelerations, combined with the measurement of fetal movements are the usual methods for identifying the status of the baby. These methods of measurement would obviously only offer a general indicator of fetal status rather than offering any information about the specific cause of such an effect. More invasive investigation during research could not only increase the risk of harm to either mother or baby, it may directly contradict good ethical practice which dictates that the well being of the subject takes precedence over the interests of science and society.
A second consideration must be that most of the research carried out has focussed on working with active healthy mothers with no complications or athletes of a competitive level. This means that it may not apply to a previously untrained individual.
Finally, we must consider the size of these studies. Many studies have only consisted of a few participants (in many cases, there have only been six to 15 participants). This poses the question of how much faith can we put in such small specialized studies? Are they representative of the general population?
So onto the research that is out there. Kemp, Greer and Wolfe (1997) compared nine healthy, active pregnant women in late gestation to 14 healthy, active, non pregnant women, and they analyzed the levels of lactic acid in the blood after high intensity exercise. They found there to be little significant difference in blood lactate between each group. The general fetal response in this study was that the fetal heart rate increased moderately and returned to normal in most instances within 30 minutes of cessation of the exercise. All the women had normal births and no apparent complications.
The effects to the fetus in the above study mimic results other studies have noted. In fact, a later study performed by Glen et al (2003) demonstrated similar findings. In this study, 17 women in late gestation performed 40 minutes of cycling at 85 percent of their measured ventilatory anaerobic threshold (approximately 138bpm), and it is worth noting that in this study, there were also no signs of any growth restrictions caused by the exercise. Although lactic acid levels were not directly measured in this study, it can be assumed that levels would be similar to the previous investigation as intensity levels were similar.
Although the above information seems to point to the fact that there are very few risks at all when performing high intensity exercise during pregnancy, please note that other studies have noted various complications with exercise of this nature. In a few cases, bradycardia (which is though to be a reflex response to hypoxia) has been experienced, and the American College of Obstetricians and Gynecologists have noted that bradycardia has been reported in 8.9 percent of cases, which actually demonstrates similar statistics to those noted in a study performed by Manders et al in 1997. Whether this is due to a rise in fetal blood lactate levels or not is unclear; however, any mother has to weigh the risks and the benefits before undertaking any form of activity during pregnancy.
As I stated earlier, it would be unethical to measure fetal blood lactate levels due to an increased risk to the baby. Therefore, the only conclusions we can make are based on a certain amount of supposition. This supposition is unfortunately a major part of programming for the pregnant lady due to the lack of hard solid facts out there. The truth of it is that we cannot say:
- If the lactic acid can actually pass through to the baby (we do know it can get into the breast milk after birth, and baby does not seem to like the taste much), but how much can pass umbillically is unknown.
- If the lactic acid has a harmful effect on the baby.
There are so many factors at play when dealing with a pregnant lady. It is a constant balance between maintaining an optimum growing environment for the little angel and developing or maintaining fitness and health in the mother.
- Glenn.N, Davies.G, Charlesworth.S, Wolfe.L (2003) Prolonged maternal exercise in late gestation-Fetal responses. Am J Obstrtrics and Gynecology 189;6:196
- Manders.MA, Sonder.GJ, Mulder.EJ, Visser.GH (1997) The Effects of maternal exercise on fetal heart rate and movement patterns. Early Human Development 48(3):237-247
- Kemp.JG, Greer.FA, Wolfe.J (1997) Acid-base regulation after maximal exercise testing in late gestation. J Applied Physiology 82:644-651
- Jovanovic.L Kessler.A, Peterson.CM (1985) Human maternal and fetal response to graded exercise. J Applied Physiology 58:1719-1722
- Macphail et al (2000) Maximal Exercise Testing in Late Gestation: Fetal responses. Obstetrics and Gynecology 96:565-570
- ACSM (2000) Current Comment: Exercise during pregnancy [online], available at URL: www.acsm.org