It's a simple rule when it comes to good manners and public transport etiquette - always offer your seat to a pregnant woman.
Of course, while it sounds simple, it doesn't always happen that way.
Some commuters stare at their phones for the entire journey, seemingly oblivious to the fact a pregnant woman is standing nearby. Others are worried about the embarrassment caused if they incorrectly assume a woman is expecting when she's not.
There's also the fact women in their first trimester of pregnancy may not yet be showing, even though those early months are often the time when a mum-to-be is unwell and most needs a seat.
South Korean transport authorities have come up with a novel way to solve the issue, to help pregnant women take a load off their feet when commuting.
The Pink Light campaign involves pregnant women carrying sensors that activate pink lights next to priority seats on trains.
The sensors, which operate on Bluetooth technology, have six months of battery life and need to be carried outside a woman's handbag for in order for the signal to be strong enough. When a woman carrying a sensor boards a train the pink light near the priority seat becomes illuminated, alerting anyone sitting in the seat to the fact they need to vacate it.
The system was tested by 500 pregnant women travelling on the Busan-Gimhae Light Rail service in the city of Busan over a five-day period earlier this year.
"Consideration for pregnant women should prevail and they should be able to use public transportation more easily and conveniently with this policy," Busan's mayor Suh Byung-soo said, according to the BBC.
"Women should be able to use city facilities easily even when they are expecting."
The trial is not the first time authorities have come up with a plan to help pregnant women get a seat on public transport.
In the UK, Transport for London has been giving out free "Baby on board" badges since 2005. The badges are hugely popular with 130,000 being handed out each year - but they are not always well received.
"The reaction to the badge is mainly that fellow passengers either pretend to sleep suddenly or are totally focused on their phones," mum-to-be Britt Soeader told the Huffington Post earlier this year.
"Both make it easy to ignore a pregnant woman and tell yourself not to feel guilty about it, I guess."