John Lightfoot on Matthew 1:18

[When as his mother was espoused] No woman of Israel was married, unless she had been first espoused. "Before the giving of the law (saith Maimonides), if the man and the woman had agreed about marriage, he brought her into his house, and privately married her. But after the giving of the law, the Israelites were commanded, that, if any were minded to take a woman for his wife, he should receive her, first, before witnesses; and thenceforth let her be to him a wife, as it is written, If any one take a wife. This taking is one of the affirmative precepts of the law, and is called espousing." Of the manner and form of espousing, you may read till you are weary, in that tractate, and in the Talmudic tract, Kiddushin.

[Before they came together.] "In many places the man espouseth the woman; but doth not bring her home to him, but after some space of time." So the Gloss upon Maimonides.

Distinction is made by the Jewish canons, and that justly and openly, between private society or discourse between the espouser and the espoused, and the bringing of the espoused into the husband's house. Of either of the two may those words be understood, before they came together, or, rather, of them both. He had not only not brought her home to him, but he had no manner of society with her alone, beyond the canonical limits of discourse, that were allowed to unmarried persons; and yet she was found with child.

[She was found with child.] Namely, after the space of three months from her conception, when she was now returned home from her cousin Elizabeth. See Luke 1:56, and compare Genesis 38:24.

The masters of the traditions assign this space to discover a thing of that nature. "A woman (say they) who is either put away from her husband, or become a widow, neither marrieth, nor is espoused, but after ninety days: namely, that it may be known, whether she be big with child or no; and that distinction may be made between the offspring of the first husband and of the second. In like manner, a husband and wife, being made proselytes, are parted from one another for ninety days, that judgment may be made between children begotten in holiness," (that is, within the true religion; see 1 Cor 7:14) "And children begotten out of holiness."