The fact is, years of on-cue sex can erode anyone's animal instincts. Keep the pressure low: rather than wearing a stellar outfit on The Day, put your effort into looking "coincidentally sexy" at all times.
To do so you must find The Holy Grail of lingerie: something blazingly sexy, that you'd choose - even if alone - to sleep in. We know, not an easy mission. But it's a must for 2 reasons: contrived desire is not sexy, and sexiness is cumulative.
"OMG it's ovulation day
...and I'm so tired"
"Hmm, look at that cute
thing curled up in my bed"
If the item scores 4/4 you have a winner. Get multiples of it, but not too many - you should aim to have at least 14 different sets. Any less and you'll start blending in with the furniture.
We won't make specific recommendations here, since every body is so different. Just find a system that works for you and your father-to-be. Avoid the large bows, thongs, gemstones, and anything tight. Embrace the silky, the sheer, and the simple.