Many women practice exercise regimens such as weight training while they are pregnant. In fact, there was a study  that showed that the third most popular activity for pregnant women was weight training. The first two were walking and swimming laps. Of course, before you begin any weight training program, you should first consult with your doctor.
Is weight training during pregnancy safe?
There may be some medical conditions  that, if you have them, it might be better that you did not weight train let alone establish any exercise program while pregnant. Some of those conditions may include:
Thyroid function abnormality
If you have been physically active using weight training as your main exercise and have normal blood pressure, heart rate and no hypertension or heart and lung problems before pregnancy, there is usually no reason you cannot workout during your pregnancy. If you have been relatively inactive, however, first see a doctor before starting any exercise program
What are the benefits of continuing my weight training program?
Continuing a steady weight training program helps with psychological continuity but more importantly, it improves your ability to cope with the postural changes and weight gain that always takes place during pregnancy. There is a condition called lumbar lordosis which is an increase in the inward curvature of the spine. This takes place because of the extra 25 to 30 pounds of increasing forward weight during the trimesters. A comprehensive prenatal strength program can keep the muscles of your legs and back supple and toned to support the ever changing distribution of weight throughout the pregnancy.
There was one study  that consisted of expectant mothers first doing a treadmill warm-up followed by weight lifting exercises for arms, legs, abdomen and back and then some cardio. It showed that for the high exercise group, length of hospitalization, incidence of cesarean section, and APGAR scores were more favorable than in the control group.
(Note: APGAR is an acronym for Appearance (skin color), Pulse (heart rate), Grimace (reflex irritability), Activity (muscle tone), and Respiration. It involves a method of assessing the health of a newborn immediately following childbirth).
What are the risks?
One of the risks associated with any kind of exercise during pregnancy is that of becoming overheated. This is because the combination of exercise and pregnancy increases the metabolic rate causing the mother’s temperature to rise slightly. There must be adequate ventilation to keep cool.
Another thing is that after the first trimester, you should not lay down flat on your back while performing any lift. This is because any exercises done in a supine position may cause the enlarged uterus to put pressure on a major vein called the inferior (or posterior) vena cava. This is a large vein that carries de-oxygenated blood to the right atrium of the heart. Pressure on this vein may cause a decrease in cardiac output anywhere from 9-20 percent.
Another risk, if you can call it that, of weight training during pregnancy, is the possible increase in a hormone called relaxin which is supposed to cause a laxity in joint ligaments such as shoulders, hips and kneecaps to name a few. This is not actually a risk but a potential condition to be expected in the later trimesters. Painful joints are certainly not fun. (Note: Weight training does not cause the production in relaxin but resistance training may exacerbate the condition).
What kind of exercises should or should not be done?
- Do not perform maximal lifts such as squat or deadlift which will cause a spike in blood pressure. In fact, do your best to stay away from compound (multi-joint exercises) as much as possible. Concentrate more on isolation (single joint) exercises such as walking lunges, leg extensions, seated leg curls, incline flyes, bicep curls, French press, etc. Machines are better and easier than free weights.
- All exercises should feel comfortable. You may not be able to go at your normal pace so slow down. If you experience any pain or nausea, stop immediately.
- Do not hold your breath while performing any lift. Holding your breath and bearing down while lifting is called the Valsalva maneuver which causes to much internal pressure and may be dangerous to you and your baby.
- Breathe continuously through every lift.
- Weights should be light enough to do about 15 repetitions for 2 or 3 sets. Workout with weights no less than 2 and no more than 3 days a week. (Again, talk with your doctor).
- Incorporate some variation into your regimen such as prenatal yoga stretching and swimming (probably the two most excellent exercises for pregnant moms).
- Every 4 weeks, take a break from the weights for a week.
Remember that the goal is not to get PRs (personal records) during this time but to simply stay fit.
What should be the nutritional requirements?
- When you combine exercise with pregnancy, your body’s demand for energy in the form of food will be higher than it has ever been. In the first 2 trimesters, it is recommended to increase food intake by an extra 150 calories a day. In the third trimester it is double that (300) and if you exercise, you will need even more.
- Eat healthy; you are eating for two, as they say. Don’t take any diet pills.
- Be sure you drink plenty of water before during and after exercise. If you gain a lot of weight and pee a lot then at least you are well hydrated.
- Don’t take any diuretics.
Take It Easy
There was a study  that asked the question if fetal growth reductions (low birth weights) were affected by leisure time physical activities during pregnancy such as exercise. It was found that leisure time physical activity is okay; it’s long hours of hard physical work that result in lighter babies. So don’t work too hard. If you do choose to continue your regular weight training during your pregnancy, lighten up on the weights and always remember to keep breathing during your lifts. Exercise comfortably in a well ventilated area, eat well and drink a lot of water. If the weight training gets to be too much then switch to yoga and/or swimming.