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At first, when you learn that you are pregnant with #2, you think: "I've done this before" and "I know what to do" and "I won't need to buy hardly anything". But, depending on how long it's been between babies, there are things you forget, things you can't reuse, and things that the first child is still using. For example, for T, we got a convertible crib which is now in it's toddler bed form. While we won't need a new crib for #2 (the toddler bed will be converted back) we will need a new bed for T. While were on the topic, he will also need a new dresser for all his big brother sized clothes.
Other things that may or may not be reusable are clothes. Some are too stained and damaged to save, others may be gender specific. So how do you check what you really need?
My method is to pretend like this is a first baby and download an app or printout a baby registry list. You can find registry 101 lists here: http://www.thebump.com/a/registry-101. Then check off the things you know for sure you don't need to re-buy for example an infant carrier, and infant car seat, high chair, bouncer, etc., i.e. anything that is reusable and not gender specific. For items you will be passing down, adjust the listing to your needs. So I will cross off crib and write toddler bed and next to dresser, I will write "for T". These are things we will still need to buy before #2 comes around.
Some of our clothes and towels and accessories are gender neutral. So they can for sure be reused. If you find out the gender of baby #2 in advance this can help you finish off your shopping list. If your second person is the same gender as your first, there may be very little to buy in regards to clothes. If not, you can sort through what you have saved from number one and then update your registry list to reflect what you already have.
No matter what the gender, we definitely plan to have a new theme for baby #2's nursery, so bedding and changing table covers will be replaced either way. We will also be replacing our baby monitor which T broke one day by spiking the monitor across the living room...
My favorite site to register on is Amazon's Baby Registry. They just have everything. We plan on registering for a few things for T too! We want him to feel like a part of the celebration and not be left out. We will also be registering for some items at a brick and mortar store as we know some people prefer that. (I can say that I'm not one of them, but we like to accommodate.)