We arrived to Tokyo and not even a month later I got pregnant with our first baby. I had no idea how hard it can be to explore this awesome new city while being pregnant in the first trimester. As you probably know trains in Tokyo tend to be quite busy and while they do have priority sitting areas (you'll know them by the yellow color and stickers on the window), getting a seat can be close to impossible.
In the first trimester it's really hard to tell that you are pregnant, so I had a few train trips where I almost cried, because I didn't know how to ask the Japanese people for a seat. Than I discovered the Ninshin Baaji or the pregnancy badge.
It looks like this>>>
What does it do: You can hang it on your bag or jacket and it lets people on the train know that you are pregnant.
Most of the times people will gladly give up their seat when they see it, but even if they don't it gives you an easy way to tell anyone without needing to speak that you are pregnant and you need a seat. So don't be mad if you don't automatically get a seat and don't be shy to ask for one, you don't have to be a hero and travel standing if you don't feel well. Just say "Sumimasen" and show your badge.
How to get it: I think you can get the badge at any train/subway station, just find a person who works at the there and show them the picture above. That is exactly what I did without even having to say a word and without having to answer any questions. Actually I did have to answer a question, when he saw the picture the guy said "Just one?" and I said "Yes" :)
So that's it, now that you have the badge you are ready to hop on the train and explore Tokyo. Enjoy!