If someone is the bearer of bad news you might say, Don't shoot the messenger. If you have something that seems unfortunate at first but ends out wonderful you might say it's a blessing in disguise.
I'm looking for a word or phrase that is a combination of the two - a bad experience that is actually bad, but ultimately heralds something good.
For example - if a couple is trying to get pregnant, and the wife gets morning sickness. There is nothing fun about vomiting and nausea, but it potentially heralds wonderful news for the soon-to-be-parents.
Is there an expression to describe the morning sickness?