This is a question I hear quite often from my pregnant friends and customers. 

However, there really isn't a right or wrong response.  In short, my answer is "when you no longer feel comfortable wearing your pre pregnancy clothing".

Some women are lucky enough to go their entire pregnancy without having to worry about wearing maternity clothes and if you happen to have a summer pregnancy you'll know that maxi dresses are a God-send!

In my first pregnancy I put on quite a bit of weight in the first trimester and found that my zip up work trousers and skirts became uncomfortable (and unflattering!) very early on.  I soon avoided anything which needed a belt as it was just too uncomfortable around my waist.  Wearing a belt certainly didn't do my morning sickness nausea any favours either.  I'd leave the house wearing one, but by lunchtime it was back in my hand bag!  As I showed quite early, I wasn't able to wear my normal work shirts without fear of the buttons popping undone.  Yes, I could have gone up a size in my regular clothing but I tried that with various pieces and they weren't comfortable, didn't look right and gaped in all the wrong areas.

I was able to briefly extend the life of my favourite jeans with a Belly Belt which was fine for around the waist, but the thigh area became too fitted and I had to put them away for a while. 

For me, with my first baby I started wearing maternity clothes around the 13-15 week mark but with my 2nd baby I was in them straight away.  It wasn't so much because my normal clothes didn't fit, it was because I felt more comfortable wearing maternity clothes.  With two pregnancies very close together I just found that maternity clothes better suited my changing body shape.  Maternity tops, in particular, are longer in length so I felt comfortable wearing them with leggings or pants.  I always regretted not buying a proper maternity denim skirt with my first pregnancy (which was in winter) and so it was the first item I bought when I became pregnant with #2 (during summer).  I wore it to bits, and still do now. 

However, I am an advocate for pregnant women wearing as much as they can of their normal clothing before leaping into full fledged preggy clothes and I say this because:

a) it saves money; and

b) you are naturally going to feel comfortable wearing something you are already familiar with and you know looks good on you

BUT, if your clothes are starting to get too tight around your waist, if you can't do up the buttons or zipper on your favourite jeans and the belly belt extender is no longer helping, if your shirt buttons are popping open and you find your long sleeve tops or regular work pants are becoming too small then it's definitely time to consider getting some quality maternity basics.

You don't need to spend a lot of money on key pieces like basic black pants, long sleeve tees, short sleeve tees and maternity jeans.  There are some fantastic styles and colours around with excellent quality material, and at an affordable price.

You may decide to borrow pregnancy clothing from a friend or family member or perhaps you might try your luck browsing your local maternity store or an online boutique for some sales - whatever you decide be sure to check the fabric quality, so you know it will wash and wear well and, importantly, see you through the full 9 months.

Making the decision about when to transition from pre pregnancy clothing to maternity wear is completely up to you - being comfortable on the inside and out is really what is most important.