(NaturalNews) A UK study very recently determined that expectant mothers' children demonstrated better muscular strength during early childhood to the degree that their mothers had vitamin D in their blood during pregnancy. So taking vitamin D, either from sunshine or D3 cholecalciferol supplements, is a healthy recommendation. More on cholecalciferol later.
The University of Southampton took 678 women who were part of the Southampton Women's Survey and tested their blood levels of vitamin D in their late stages of pregnancy. Then, four years later, they tested their offspring's muscular strength with grip testing and measured their muscle mass.
Muscle mass was increased mildly among children whose mothers had higher vitamin D levels during pregnancy, and grip strength among those four-year-old children was dramatically greater. The higher the vitamin D levels were among the mothers, the greater the muscle mass and grip strengths of the young children. 
If you want strong kids, make sure your vitamin D level is more than adequate.
A South Korean study determined that mothers with higher vitamin D levels delivered children who were at much lower risks of respiratory tract infections (RTIs). The mother's vitamin D level helps determine the child's immune system strength. 
Sunlight exposure and supplements
The trick is to build up one's serum vitamin D level even before the fetus is feeding from the mother's cord, and also for it to build up in the mother's breast milk.
Bare skin exposure to sun or in UVB tanning beds for 15 minutes can produce between 10,000 and 20,000 IU (international units) of vitamin D3, which the body stops producing after your vitamin D levels are high enough, no matter how much tanning you do.
When the sun's UVB rays begin their conversion via skin cholesterol, cholecalciferol is created. This is the first phase of the prehormone called vitamin D, which is ultimately created by the kidneys and liver to assist with hormonal production and boost the immune system.
Vitamin D is not just for avoiding rickets. It's a healthy multitasking substance. 
Vitamin D3 cholecalciferol supplements are mostly derived from the oily lanolin (cholesterol) of sheep wool. In other words, sunlight UVB-created cholecalciferol is packaged into capsules, enabling one to supplement with the vitamin in lieu of adequate sun/skin exposure.
As we age, our sun/skin conversion factor declines. It takes four to six months of supplementing vitamin D3 for your blood levels to peak.
So how much is enough?
A couple of decades ago, most doctors were woefully ignorant of what constitutes a vitamin D deficiency. But more are catching on thanks to a handful of vitamin D researchers. One of them is Dr. Bruce Harris, MD, Ph.D, who has researched vitamin D for over 30 years.
For supplementing vitamin D, he feels that most adults should take over 2,000 IU daily. He and others consider the 600 IU RDI (recommended daily intake) woefully low and actually dangerous, as it invites long-term poor health and chronic diseases.  
Your blood level of vitamin D is determined best by the 25(OH)D blood test, also known as the 25-hydroxy vitamin D test or 25-hydroxycholecalciferol test. That's the one to use over any other test. International readings use nmols/L (nanomols per liter), while the USA uses ng/mL (nanograms per milliliter).
The conversion factor is 2.5. Use 2.5 as a divisor to change a ng/mL reading to nmol/L. Use 2.5 as a multiplier when you want the opposite conversion.
Here are the currently established 25(OH)D reading standards in ng/mL (nanograms per milliliter): 
Under 30 - Deficient
30 to 50 - Insufficient to adequate
50 to 70 - Optimal
70 to 100 - Therapeutic for heart disease, cancer and other diseases