Guide to Bras Throughout Nursing and Pregnancy

I wrote this for /r/ABraThatFits, with the help of the wonderful t_maia.

Pregnancy 

You don’t have to wear a maternity bra during your pregnancy. If there aren’t any made in your size, you hate the styles offered, or they are too expensive for you, just buy regular bras or nursing bras. This should probably be obvious, but isn’t, because advertising people are really good at their jobs. You do not need to wear wireless bras throughout pregnancy. The problems many pregnancy books mention are caused by the fact that too many women wear wired bras in the wrong size. If the cups are too small or too narrow, the wires will sit on breast tissue, which can be highly uncomfortable for pregnant women and interfere with milk production. Please measure yourself according to the sources in the sidebar if you haven't already.

If you do buy unwired bras, buy nursing bras with clips instead of maternity bras without clips. The bras you'll buy during the second trimester of pregnancy won't fit you right after birth, but will usually fit again after a few months postpartum. It is a bit of a gamble, but one that tends to work more often than not. So the bras you buy during pregnancy do not need to be cheap throwaway bras.

Read reviews to see if the bras score high on comfort, because nothing is worse than being pregnant and uncomfortable. Most books say you grow in the first trimester and then stop, or only grow a little more after that. That’s true for some people, but not for everybody. Quite a lot of women grow in the second and the third trimester too.

Bras that you buy during the first two trimesters of pregnancy should fit you ok on the tightest or middle hook. (This is opposite of the regular fitting advice to buy bras that fit ok on the widest hook.) As your ribcage expands you can switch to the wider hooks or use extenders. Many good nursing bras come with more than two or three columns of hooks for this purpose; four to six is common. As long as you need only a wider band, extenders work fine. Remember that you can daisy-chain extenders if your ribcage expands more than a single extender can accommodate. But if you need a larger cup, you'll need to buy new bras.

Some sources will recommend stretchy bras that will allow for some fluctuation in size. Crop top bras from companies like Medela are popular for this. But many women with larger cup sizes find them unsupportive; the stretchy fabric allows too much room for the sensitive breast tissue to move which can be highly uncomfortable.

During the very last stages of pregnancy many women find underwire bras uncomfortable, because the wires dig into the belly. This can be especially a problem for women with breach babies who carry high. Some nursing bra makers use relatively soft wires in their bras specifically for this purpose, but it is often at this stage that many switch to non-wired bras.

If you are seeking to order nursing bras one or two weeks before your due date, note it is a bit unpredictable to say what size you'll be after giving birth. But try a band size down and a few cup sizes up from what you measure. Say you measure currently as a 34F UK, you'll need a band size down (=32FF UK) and 2-3 cup sizes up, so 32GG/H UK should do the trick.

The First 6 Weeks Postpartum  
Most books recommended not wearing bras with underwire until your milk supply has fully regulated. I disagree. The reason given in the books is the wires interfere with milk ducts, but this is only true for too small bras. It should not happen if you wear the right shape in the right size. In fact, wearing an underwire bra might have saved me a lot of pain when my daughter and I were getting started breastfeeding.

I only brought a sleep bra to the hospital with me. Newborns want to nurse all the time. They are still learning, and you are still learning, so it is a lot of work for very little payoff at first. The night before my milk came in, my daughter nursed 20 minutes on, 20 off for 10 straight hours. It’s not worth fiddling with a bra while you are waiting to produce milk instead of colostrum. Engorgement, however, is another story. Soon after your milk comes in, you will find that your boobs are massive and rock hard. They will seriously weigh more than you thought they ever could.

If you are larger than average, the weight of them can actually block your milk ducts- and this is what I meant when I said if I had a properly supportive bra, I might have avoided some pain. I got mastitis (an infection, most frequently caused by blocked ducts) three times in the first two months of my baby’s life. Mastitis feels like the flu, only worse, because your breasts hurt and you have a screaming baby to take care of. Wearing underwires that are too narrow is a near guarantee that you will get mastitis. Err on the side of caution here. The wires need to be well clear of your breast tissue. You will also want a bra that is either very open on top or has stretchy lace to accommodate for sudden change in size. Wearing a too-small bra is also very likely to result in mastitis; bras that reshape your breasts against its natural shape and bend the milk ducts can have the same effect. (Ewa Michalak nursing bras are notorious for this, which is why they are not recommended for mothers who are not at least 6 months post-partum.)

The first couple months of breastfeeding, your breasts will fluctuate sizes fairly rapidly. At this stage, milk production is largely hormonal (not supply-and-demand like later on). It is not worth really stocking up on bras until your size has more or less settled - usually around three months.

Three months to six months  
At this point, your nursing relationship should not be as stressful anymore. You are probably pretty confident in your ability to nurture your baby with your body, even if things started out a little difficult. Or you’ve settled into a routine of splitting formula/breastmilk, which is also fine! But your breasts are finally getting to where they will stay for a while. You can now measure yourself again using the resources in the sidebar. If you find you are in the 32-40 C-F range, you will probably not have any problems finding nursing bras that fit you, and cute ones at that! If you’re sized out of that, though, you still have options. Any bra can be converted into a nursing bra. If you are on the smaller side, you can generally slide the strap off your shoulder and pull your breast out of the cup to nurse. I do this with both soft cup and padded cup bras with no problem. (I did, however, tear a Freya Deco, so I don’t recommend doing this with moulded cup bras.)

Six months to a year  
Remeasure yourself at six months, even if it seems like your three month bras are still working. If nothing else, it’s interesting to see if the numbers have changed at all. Depending on if you wore ill-fitting bras before or during pregnancy, you may have experienced tissue remigration by now, and could end up sizing down in band and/or up in cup size. As long as you are nursing, measuring every three months is a good idea, but from this point on your size will probably stay about the same.

One year to weaned
First, congratulate yourself on making it this far. You have provided the best possible nourishment for your baby! Around one year old, your child will probably eat more solid foods and nurse less often- three or four times a day, then two, then maybe only at night. Keep remeasuring yourself every three months or as needed. Nurse as long as you and your child want to, keeping in mind that left to their own devices, children usually wean completely between 2 and 7 years of age and it is perfectly healthy to keep going as long as you want. Once your child is completely weaned, it can take up to a year to completely dry up. You will likely return to your pre-pregnancy size, give or take a cup size (even if you are shaped a little differently now).

Other resources:
/r/breastfeeding
Convert any bra into a nursing bra
Roundup of bras outside the Motherhood Maternity size range 
Some myths about maternity and nursing bras