Verse 13: So Boaz (Ruth 3:11) took Ruth, and she was his wife: and when he went in unto her, (Gen. 29:31; 33:5) the LORD gave her conception, and she bare a son.
[Boaz took Ruth] Question: How was he able to take a foreign woman? Response: Because she had already been converted to God, and there was no danger of leading her husband astray; which was the sole reason for the prohibition of marriage with such (thus Munster).
Boaz took Ruth; which he might do, though she was a Moabite, because the prohibition against marrying such is to be restrained to those who continue in the heathenish estate, as is evident from the reason of it; whereas Ruth was a sincere proselyte and convert to the God and faith of Israel. He went in unto her, that is, had conjugal converse with her. See Genesis 6:4.
[He granted…that she might conceive (thus Vatablus, Tirinus), הֵרָיוֹן] A fœtus, or conception (Septuagint, Jonathan, Pagnine, Montanus).
Conception, that is, strength to conceive and retain seed.
 Hebrew: וַיִּקַּ֙ח בֹּ֤עַז אֶת־רוּת֙ וַתְּהִי־ל֣וֹ לְאִשָּׁ֔ה וַיָּבֹ֖א אֵלֶ֑יהָ וַיִּתֵּ֙ן יְהוָ֥ה לָ֛הּ הֵרָי֖וֹן וַתֵּ֥לֶד בֵּֽן׃