The command for Manoah’s wife to be careful
here has the sense of, Guard yourself, that you keep this restriction during your pregnancy.
Notably, this particular form of the verb (in the niphal) occurs frequently in Deuteronomic commands for diligence in the observance of the divine will (see Deuteronomy 2:4; Deuteronomy 4:9, Deuteronomy 4:15; Deuteronomy 6:12; Deuteronomy 8:11; Deuteronomy 12:13, Deuteronomy 12:19, Deuteronomy 12:30; Deuteronomy 15:9; Deuteronomy 24:8). The role of Manoah’s wife is not a passive one.1
Samson of course fails to guard his Nazirite status and calling. He plays with it.
4 Therefore be careful and drink no wine or strong drink, and eat nothing unclean,