The text uses four verbs – paru vayishr’tzu vayir’bu vaya’atz’mu: “they were fruitful, they swarmed, they multiplied, they became (numerically) powerful” (Ex. 1:7).
It is possible that the four verbs are more or less synonyms, introduced for the sake of emphasis.
Rashbam does not agree with this view, but argues that the verbs are piled up to indicate a series of stages.
“They were fruitful” means that pregnancies continued to occur. “They swarmed” indicates that the pregnancies were productive and the babies were actually born. “They multiplied” denotes that the infants survived. “They became powerful” tells us that the children grew into manhood and did not die early.
This interpretation explains why the Egyptians became so apprehensive. In contrast to the prevailing trends, the Israelite women brought their babies to full term, there was little infant mortality, children survived into adolescence and the young turned into healthy adults.
Was it Israelite hygiene that brought such benefits? We cannot be certain about the health measures that prevailed prior to the giving of the Torah, but it may be that there was an innate sense of cleanliness.
Later in history there were Torah guidelines to live by and very few Jews succumbed to disease or epidemics. In the Middle Ages antisemites accused Jews of poisoning the wells and causing the plagues that devastated many lands; the truth is that Jews lived such a careful, hygienic life that they were able to ward off infection.