It is an unfortunate evolutionary fact that women become fertile several years before what is, for mother and child, the safest time for birth; moreover, they usually remain fertile for ten to fifteen years beyond the period of lowest risk. While the onset of fertility ranges from age 10 to the mid-tens, pregnancy becomes safest from a biological point of view around the age of 20. The period of maximum safety lasts for about a decade; then, when a woman reaches the age of about 30, risks to mother and child begin to rise and they continue to escalate with each passing year.
The number of children a woman bears in her life affects her health significantly. Her first birth carries a slightly higher risk of complications or death for her and her child than second and third births do, primarily because the first birth reveals any physical weaknesses of genetic abnormalities in the mother or the father. A woman's second and third births are generally the safest but with the fourth birth, the incidences of maternal death, stillbirth, and infant and even childhood mortality begin to rise, jumping sharply with the birth of the fifth and every succeeding child. Beyond a certain point, then, practice does not make perfect in childbearing; quite the contrary, it entails escalating dangers. The actual level of risk involved in bearing large numbers of children depends, of course, on the mother's social milieu. But one pattern prevails in every country and in every social class: risks increase as the number of children passes 3. Contrary to the belief held by many people, including some doctors, that women with many children are apt to give birth easily and painlessly, such women are in fact particularly susceptible to the complications and diseases associated with pregnancy.
Some studies found that about 60 percent of all severely undernourished children were of the fourth or later birth order. It is calculated that even without any other improvements in income, food availability, or medical care, the "limitation of family size to 3 children would bring down the incidence of severe forms of protein calorie malnutrition by at least 60 percent". The ill effects of numerous births on both mothers and their children are more likely to occur when the intervals between these births are short. Studies have shown that infants born less than two years after the previous child are 50 percent more likely to die by age 1 than are infants born two to four years after the previous child. Considering the health of mother and children, the risks related to the reproductive process could be reduced to the lowest level possible if:
1. The birth of the first-born can be deferred until the mother is 20 years old 2. The space between children's births must be at least 2 years 3. No more than 2-3 children are to be born 4. No more births after the mother's age reaches 30 years or over should be expected. "