Congratulations on your pregnancy, I do hope you are feeling well! I like pregnancy, well apart from the nausea and chronic reflux, but thankfully the nausea subsides and the reflux can be treated.
I gained about 10-12kg during my pregnancies, and this extra weight gain meant I required some bigger underpants. I thought it would be useful to provide a guide on how to buy the right underpants size based on kilograms gained.
The average woman gains 1 to 2 kg in the first three months, followed by 1 to 2 kg per month during the final six months (www.thewomens.org.au/Weightgaininpregnancy).
Based on these averages, after 16 weeks you may have gained approximately 2 to 5kg, which is equivalent to one half to a full size. Not all women will feel the need to purchase a larger size at this point. After 32 weeks, you may have gained approximately 5 to 10kg, which is equivalent to 1 to 2 sizes.
Most women know their bodies and will have a good idea of how much weight they are gaining. I recommend my customers and pregnant friends purchase a couple of pairs of underpants 1 size bigger at the beginning of their pregnancy and then a few more pairs at about 5-6 months, 1 to 2 sizes bigger. I would also suggest buying Hipster styles so the underwear can sit comfortably under your baby bump.
In terms of how many pairs to buy, I would say enough to see you through this and future pregnancies, but not too many as to stop the incentive of losing that extra baby weight.
Good luck ladies!