Skip bras with wires to allow your breasts to develop during pregnancy and early breastfeeding stage.
Nursing bras make nursing more convenient.
Shop for Asian brands since they cater more for our small built.
I bought my bras from Fabulousmom.com (based in Malaysia).
Moms in Mind (Singapore) may have some offerings too.
When milk comes in after birth, cup size may increase a bit. After baby finishes his feed, size may be back to normal again.
There are also nursing tank tops (e.g. From Spring Maternity Suntec) that goes by S, M, L. I'm very petite and fits into M, so you may find a suitable size.
Enjoy your pregnancy and breastfeeding journey!