Many people ask this question and have many answers; But it is not easy to be "scientific".
To be scientific, we must first strictly define what we want to describe, and the complexity of this problem is that it is difficult to define "reasonable" or "good".
From the child's point of view or from the mother's point of view?
From the child's point of view, is it to examine the child's IQ or EQ, or psychological development and personal achievements? Don't make a strict definition, just talk casually.
In order to answer this question scientifically, at the end of last century, a far-reaching article was published in the New England Journal, which gave the answer to this question from the perspective of newborn health.
From the perspective of newborn health, gestational age and birth weight are two important indicators. These results are closely related to the health of children after birth and the health of children and adolescents. Therefore, this paper focuses on these two indicators: premature birth risk and low birth weight risk.
In this paper, more than 70,000 newborns delivered in Utah in the past eight years have been studied. Finally, it is concluded that it is the most ideal situation to get pregnant again after the previous birth 1.5 to 2 years only from the perspective of gestational age and birth weight of newborns.
The study found an interesting "J-shaped curve", that is, the risk of premature delivery and low birth weight decreased gradually with the increase of the interval between two fetuses, reached the lowest risk at 1.5 to 2 years, and then increased with the increase of the interval between two fetuses.
J curve
(Source: Influence of pregnancy interval on perinatal outcome. )
Specifically, compared with the pregnancy after 1.5 to 2 years, the risk of interval less than half a year is about 1.4 times, and the risk of interval more than 10 years is about 1.5-2 times.
This article is widely cited because of its large sample size and excellent experimental design. The conclusion of this article is still used in the latest edition of evidence-based guidelines for obstetrics.
In addition, from the mother's point of view, there are many literatures about the interval of the second pregnancy after cesarean section, and the conclusion is the same, that is, the incidence of pregnancy and childbirth complications is relatively low if it exceeds 1.5 years. Similarly, when the interval is too long, the risk of complications will increase. For example, I have done research in this field, and found that the risk of complications of scarred uterus increased significantly when the interval was more than 13 years.
Therefore, based on the above data, regardless of other social, humanistic and psychological factors, only from the perspective of perinatal health, 5 to 2 years after giving birth to the first child/kloc-0 is the ideal interval for pregnant with the second child.
Although this "J curve" shows an ideal time period, this interval is not so dogmatic, and the task of pregnancy must be completed within half a year.