Pregnancy and Bone Density: Impact on Women's Health

Question
Question; What part dues pregnancy/nursing play in bone density, particularly in thinner, smaller women? Can having children very close in age and nursing negatively affect bone density?

Background: I am a 36 year old female 113 non-pregnant weight; 5 3 height, now 14 weeks pregnant. My 3 other children (2, 3, and 4) were all born within 2 1/2 years of each other. After having the three children, I immediately got back into a running/work out routine. After a few months I had severe pain and was diagnosed with 2 pubic ramus stress reactions/fractures. A DEXA scan determined that my bone density was very low (osteoporosis) in hips. Many tests were run but no cause was found. Now 14 weeks pregnant, I am exercising but worries about my bone density .

Answer
You have a complicated situation.
In theory, the demands of pregnancy decrease maternal calcium stores which may account for the relatively rare cases of osteoporosis arising in pregnancy. On the flip side, pregnancy hormones increase our ability to absorb calicum which should guard against this complication. So it is unclear why some women develop this problem. Genetics may play a role, as well as some risk factors, such as the use of certain medications. But the science of bone remodeling in pregnancy is not yet clearly understood.
I suggest that you seek expert nutritional support, so that you can be sure that you are getting enough calcium, magnesium, and Vitamin D to try to off set your condition.
As for exercise, don't do any impact exercise. Safe for you will be fitness walking, elliptical, recumbent bike, swimming.
The best osteoporosis rehab exercise to load the hips is the standing vertical press. Use free weights, at an easy enough weight where you can easily control your core (spine neutral, ribs don't flare, and the pathways of the weights). Hold the weights at shoulder level, and press directly upward, lightly bringing the weights together over your head. Hold the position for 10 seconds (this is when your hips are being loaded) then lower down. Since you're pregnant, don't progress to one arm, or alternating extensions, as you do not want to challenge your core to control an asymmetrical movement.