Does “a woman” in Leviticus 18:19 and 20:18 refer to all women including a man's wife or just women who were not his wife.
Yes , it includes all women including his Wife.
If during intercourse a woman inadvertently has a menstrual discharge, her husband was unclean for seven days, the same as his wife:
Leviticus 15:24 (NASB)
24 "If a man actually lies with her so that her menstrual impurity is
on him, he shall be unclean seven days, and every bed on which he lies
shall be unclean."
If they continued sexual relationship whilst she was menstruating ,thus showing contempt for God's Law, the penalty of death was imposed on both, husband and wife.
"If there is a man" means anyone including her husband.
Leviticus 20:18 Bible (NASB)
18 "If there is a man who lies with a menstruous woman and uncovers
her nakedness, he has laid bare her flow, and she has exposed the
flow of her blood; thus both of them shall be cut off from among their
For the above reasons, when ceremonial cleanness was required, as, for example, when men were sanctified for a military expedition, they were obliged to refrain from having intercourse with their wives.1 Samuel 21:4- 5 ; 2 Samuel 11:8-11.