Working Through The First Years Sterilizer Instructions
The First Years brand made a microwave sterilizer bag system that was pretty common in the mid-2010s. If you have one sitting in your cabinet and can't find the guide, here is what you need to know about running it. These are basically instructions for a microwave-safe bag with a measuring cap for water. You put your bottles, nipples, and small parts inside, add a specific amount of water, and zap it in the microwave. The steam does the rest. The process takes about three minutes per cycle, though it varies by microwave wattage. I ran these for about two years straight during my kid's bottle days. They work fine if you follow the water measurement carefully. The biggest issue people have is eyeballing the water instead of using the little cap that comes with it. Pour too much and you get wet bibs of condensation everywhere inside the bag. Pour too little and the steam cycle is weak and won't actually sterilize properly.
Here is the basic procedure: Disassemble everything. Separate the bottles from the nipples and rings. You need all the surfaces exposed to steam. Put the items in the bag upright when possible. Close the vent flap after adding water. Microwave on high for the recommended time. Let it sit for a couple minutes after the cycle ends before opening the bag because the steam inside is hot enough to burn your hands if you pull the flap back immediately. One thing the instructions don't emphasize enough is that the bag gets extremely hot to the touch even after the microwave stops. I once pulled the bag out by the side and set it on a cold granite counter, then grabbed it again thirty seconds later thinking it was cool. It wasn't. The plastic retains heat for several minutes. Let it sit on a silicone mat or towel, not directly on stone or laminate.
Another practical note: these bags are not designed for large breast pump flanges or wide-set valve assemblies. The interior space is cramped. I found that standard bottles and nipples fit easily but anything wider than a typical bottle neck barely clears the top seal. If you're pumping, you might need a different sterilization method for the larger parts. There are also limitations worth knowing. This is a microwave steam system, not an ultraviolet or electric dry-heat unit. That means the sterilization relies entirely on consistent steam generation. If your microwave runs at a lower wattage than the instructions assume, the cycle time may need adjustment. My microwave was on the weaker end at 700 watts and the three-minute setting left some condensation pools at the bottom of the bag. I ended up doing a second thirty-second burst to compensate. The bags themselves are single-use in practice. You can technically reuse them a few times but the material degrades. The seals weaken after repeated heating cycles and the plastic starts to look cloudy and brittle. I stopped reusing a bag after about six to eight cycles. Once the flap doesn't seal shut on its own anymore, it is time to replace the whole unit. You can find replacement bags reasonably cheap at most baby stores and online.
Get the Full Details
If you want the official documentation, The First Years has historically hosted their product manuals on their parent company Dream On Me website. You can search for your specific model number on their support or products page. The model is usually printed on the back of the bag or on the original packaging. Common model numbers include the FS100 series. The manual PDF walks through the water measurement markings on the cap, which are labeled for different microwave wattage ranges. A couple of counter-intuitive things I picked up after months of use. First, you do not need to pre-rinse bottles thoroughly before loading them into the bag. A quick water rinse to remove milk residue is enough. The steam cycle handles sanitizing, not heavy food removal. Second, turning the nipples inside out before placing them in the bag actually helps. The steam reaches the inner surface that way, and it prevents that weird pool of water that sometimes collects inside a nipple after the cycle. The main downside of this system compared to a dedicated electric sterilizer is the lack of a drying phase. Everything comes out damp. You either leave the bag open until things air dry, which takes twenty to thirty minutes, or you use a clean towel to pat things down. If you are in a rush and need dry bottles immediately, this isn't the tool for that. An electric UV sterilizer with a drying function will serve you better in those situations, though they run about two to three times the price.
I kept these around for convenience during the early months when I was doing frequent microwave cycles throughout the day. They worked. They were not fancy. They did exactly what the instructions said as long as you measured the water and gave the bag time to cool before handling. That is pretty much it.
