People could sometimes be insensitive that they would just blurt out questions that they won't think it could be offensive for some, especially for a pregnant woman. You might mean well with your inquiry, but it doesn't mean the other person won't get mad about it.
In case you don't know, but pregnant women could be very sensitive and highly emotional since they are raging in hormones due to their current situation. They are easily ticked out if they hear something that sounds offensive to them. A mother wrote an essay via Romper sharing how she was annoyed whenever people asked her if it was planned a pregnancy when she was pregnant with her middle son just proved one must be very careful with their questions.
Most women find it offensive, not to mention invasive, whenever people asked them if the one they are carrying inside is a result of planned pregnancy. It could sound like it is a reasonable question but why does it tick them off? A British study published on NCBI explained the definition of planned pregnancy and based on their paper; it means an intention to become pregnant and ceasing the use of contraception.
In a pregnant woman's mind, there is also the opposite of the term which is unplanned pregnancy or unwanted. That is the part when they could be offended because it does sound like there is a chance that they don't like the child that they are bearing. This is why it is safe never to bring this question up if you ever talk to someone who is pregnant.
The mother from Romper's essay also made a good point when she said that her life in the bedroom is no one's business anyway, so it is entirely inappropriate for anyone to ask if her pregnancy was planned. Now, you know what not to ask a pregnant woman when you see one.