Good question. As pointed out before there are quite a few seats which haven't gone through the Plus Test. If you don't want to read the whole thing below I'll point out that DuoLogic is the only seat which will be a great fit for a 7 week old. It will last to around 4 years of age.
Short background of the Plus Test (likely far more than anyone wants to know;-):
The new standard is by far the strictest car seat standard in the world. We have created this one in order to make up for some shortcomings in the European ECE R44 car seat standard. The ECE R44 is still good and strict, a bit stricter than the US FMVSS 213 standard.
The main weakness with ECE R44, like other standards, are all the forward facing seats for young children. The difference in safety between rear facing and forward facing seats is simply huge, 500% or five times safer in favor of rear facing. Parents should not use forward facing seats before age 4 but many are still unaware of this crucial information.
Here in Sweden we talk a lot about keeping kids rear facing until age 4 or longer. We feel very strongly about not using forward facing seats for children until then. There is no law about minimum rear facing time in Sweden, I could theoretically FF my child at 7 months, but parents are in general quite well informed about rear facing.
We don't just talk about no forward facing seats, they are also not sold in the country. Trying to buy a forward facing seats for a 2 year old child is basically impossible since none are sold. There is no law about this, retailers simply don't sell the FF seats since we care very much about our children staying safe in traffic.
Sweden has been way ahead in car seat safety for a very long time. In 1975 we created the excellent "T-standard" which was a good car seat standard. By then we had already been uysing rear facing seats for 10 years. Back then there was no advanced equipment available to measure forces in the neck like today, it was done through measuring in the chest area etc. The T-standard was phased out in 2008 due to EU regulations, only one standard is allowed in Europe.
The Plus Test is measuring forces in the neck which makes it impossible for any forward facing seat to pass the test. This is exactly what we want since neck/head forces in a forward facing seat are extremely high and dangerous for young children.
The Plus Test is also using a higher crash speed than the EU or US standard and a different crash test dummy. The "Q" dummy is used which is incredibly expensive. The overall crash pulse becomes different in the Plus Test and makes it far more difficult to pass.
Seats such as Britax Multi Tech and Britax Two-Way can be used rear facing and forward facing for older children. Since the the seats can be used forward facing, regardless if it's when a child is 5, they must also be tested this way. Forward facing thankfully have no chance of passing the Plus Test so these seats are not even tested. Multi Tech and Two-Way are still just as safe rear facing as seats which have passed the Plus Test
None of the many popular forward facing seats sold for very young children in UK, France, Germany, Ireland, etc. go through Plus Test since they have no chance of passing due to the off the chart high forces in head/neck.
Most of the Swedish rear facing seats have passed the Plus Test:
- Britax Hi-Way and Belogic are the only ones which have passed up to 25 kg (gold standard)
- BeSafe X3, X3 Isofix, and DuoLogic have passed up to 18 kg (silver standard). 18 kg is the maximum rear facing weight for any Isofix rear facing seat.
Maxi Cosi have said they will not currently test the "Mobi" model. This is still an excellent seat which is just as safe as the other seats. (It also have the best recline/sleep position of any RF seat).