Append a placeholder value to a text input field when form is submitted
You could either do this the JavaScript way (listen to onSubmit, grab DOM values, append if necessary, otherwise remove), or the PHP way (from a plugin).

I'll pseudocode the latter because I hate the former.


Generate a Craft plugin.
Have it listen for the onBeforeSaveEntry event.
Inside that event listener, you can check if the entry being saved belongs to the section you're interested in.
If so, check your Fetal Heart Rate field.
If its value is "bpm", set it to null or an empty string.
If it's anything other than "bpm", append " bpm" to its existing value.
Repeat 4-6 for the "Estimated Gestational Age" field.
Bob's your uncle.