Madhurashtakam, the hymn that begins 'Adharam madhuram', is credited to the saint Vallabhacharya. In eight short verses it calls everything about Krishna sweet: his smile, his flute, his play, even the dust under his feet. Every line comes back to the same word, madhuram, so it sings like a lullaby. That is why many mothers choose it in pregnancy. You can sing it in any month, and there are no special rules.
Pick one simple tune and keep to it. Sing one or two verses in the evening, in a quiet room with a hand on your belly, or all eight together as one song. Someone close to you and older children can join in. Research has found that babies may start to respond to sound from about 19 weeks, that they can learn sounds they hear often late in pregnancy, and that newborns prefer their mother's voice to another woman's. So your baby may find the tune you sing now familiar after birth. Many parents keep singing it at bedtime after the baby arrives.
An honest note: babies may learn the sound of a song, not its meaning, and singing won't change a baby's health or development. It is a sweet ritual you can share. If you ever feel unwell or worried, call your doctor first. Read our disclaimer