Well, it can, but it is not a definite indication of pregnancy. The only two definite indicators for pregnancy are ultrasound or birth. 🙂
We have watched for indicators on our Large Blacks and have found that most times the “light switch” does ring true for pregnant sows. We also look for erect nipples when the sow or gilt in question stands up from lying down. When she is lying down, the nipples get pressed close to her body and when she stands up, if she is not pregnant, the nipples will remain “sticking” to her belly but if she is pregnant, they will be erect when she gets up. We also look for the “keyhole” effect. Looking head on a pig, if they appear to be an inverted old fashioned key hole (narrower at top with a definite roundness through the lower belly), then it is more likely that they are pregnant. I am sure others have more Old Wive’s Tales to share, but these are ours.
We do rely heavily on ultrasounds from our vet to confirm what we believe. Sometimes the Old Wive’s Tales turn out to be correct and sometimes they do not. And once in a while you will get a gilt that doesn’t look pregnant at all and shouldn’t be pregnant and poof, there are babies when you are not expecting them!! It is fun to watch for those first indications of pregnancy. I use them, but if I want a definite answer, I call my vet or simply wait.