Numbers 3:43 states that in the second year after the Exodus there were 22,273 firstborns over the age of one month. If you add 300 for the firstborn Levites, you arrive at approximately 23K firstborns. Out of a population of 600K men between the ages of 20 and 60, and presumably 600K women of that same age group, why are there only 23K firstborn males? One would expect that half of the 600K couples would have a firstborn son, which would imply that there should be 300K firstborns. Why is there less than 10% of that number?
R. Aryeh Kaplan, in his footnotes to The Living Torah, suggests that many Jews in Egypt did not put blood on their doorposts during the final plague and therefore their firstborns were killed as well. Alternatively, he proposes that many women gave birth to girls first, but does not explain why that might be the case.
R. Yehuda Henkin, in his New Interpretations on the Parsha (p. 117), offers the following explanations:
Perhaps initial miscarriages resulting from slavery, malnutrition, and child marriages were prevalent in Egypt, and these denied the status of firstborn to subsequent live births. Or, there may have been few firstborn in the first census in the desert because few elected to leave Egypt, where they were a privileged class.
The Midrash offers another possibility. The Torah employs six verbs to describe the fecundity of Israel in Egypt (Shemot 1:7), and the Midrash sees this as an allusion that Jewish women gave birth to sextuplets. If so, ten pregnancies per mother would yield sixty offspring, of which only one would be firstborn; half of the children and of the firstborn would be males, with one male firstborn for every sixty male births.
 Cf. Shemot Rabbah 14:3. Similarly, in the first census there were only 22,000 male Levites aged one month and older, as compared with 32,200 male adults in the next smallest tribe, Menashe. Perhaps Levites, too, were a privileged class in Egypt and few of them joined the Exodus; see ibid., 5:16. Alternatively, their higher status corresponded with a drop in fecundity; cf. Shemot 1:12.  Shemot Rabbah 1:8.