The story
From a song, "When You Were a Smile on Your Mother's Lips (and a Twinkle in Your Daddy's Eye)", written for the 1934 film, Dames; derived from the presumption that prior to the conception of a baby, the father was sexually aroused, and this arousal was reflected in a pleased look in the father's eyes.
Etymology from Wiktionary, CC BY-SA 4.0.