"Showing" occurs at different intervals for people, and while it's hard not to compare pregnant bellies, it really is a good policy to avoid comparison. A mother who has had previous children will likely show more quickly in her second pregnancy (or more) than a mother pregnant for the first time. Also weight gain, previous weight status (slim, normal, overweight), whether or not you've had previous abdominal surgery, the tone of your muscles etc. will all play into account. That said, many women start to feel the need to be out of their regular clothes by the 3-4 month range. This doesn't necessarily mean maternity clothes for everyone at this point.
Earlier in pregnancy when you first show, you may notice that how big your belly looks depends a lot on various factors, such as is it morning? Have you eaten a lot? I would often notice that I wasn't showing when I got out of the shower in the morning, but by the evening, I looked huge. I assumed it was my muscles being more relaxed at the end of the day. Whether it was a big dinner or lax muscle, the fact was I looked more pregnant at the end of the day than at the beginning.
Here's what other moms had to say about showing:
"I was on the skinny side, so I felt like I was showing at six weeks. While I wasn't wearing maternity clothes, my mother asked me if I was pregnant when I was eight weeks along. No more hiding it, if you knew me and my flat abs - the gig was up."
"I started my pregnancy about ten pounds overweight, but it was more muscle. I was very athletic and lifted weights. I had great core strength and I think that caused me to hide the pregnancy a bit better. While my husband could tell I was pregnant when I was naked, at around 17 weeks, no one else could tell until almost my third trimester. I was still in great shape after having my first, so my second I expected to be much the same. I showed around 22 weeks, which still wasn't bad, but made me feel a bit chubby."
"My pregnancy was a surprise, so I wasn't planning to get pregnant and wasn't in great shape. I wasn't awful but didn't work out often either. I also started eating a lot right away because that was the only way that I could keep morning sickness at bay. So the eating and the being in bad shape didn't do me any favors. I think I broke out the maternity clothes at five months, but you could tell I was showing a bit before then. My boy friend saw me naked and said he noticed sooner, like around twelve weeks."
What partners have to say about a belly showing or not:
"My wife was so excited about being pregnant. She bought maternity clothes really early, even though she didn't wear them. I think some of her outfits were cut oddly and that made her look bigger than she was. This was fun for her early on, but later as you could really tell that she was pregnant, it was no longer any fun."
"I loved looking at the rounded abdomen. It started out with just a slight bulge. No one could tell but us in private times. When she showed even while dressed it was a bit sad for me at first because I felt like our private secret was out."
"I was so excited when she showed. I wanted to scream - 'That's my baby in there!'"
My belly grew faster than her belly - it was so sad. I couldn't believe it and wasn't sure how I gained weight so quickly along with her. I think I started showing at the fourth month and she didn't show until the fifth. Thankfully neither of us got stretch marks!"
"I wasn't as observant as she would have liked. I know it made her angry but I just wasn't as into it as she was in the beginning. Every night, she'd stand in front of the mirror and ask if she looked pregnant yet. I felt like no matter what I answered, I was wrong."