Need to transition to a new carseat

Question

My baby is almost 5 months, and he's 20 pounds almost,  and don't fit in his carseat anymore.
I need a recommendation for a good carseat to buy.

Answer

**Since your child is already proving to follow a higher percentile track for weight with his growth, PLEASE do your research and looks for a seat with a 50lb rear facing limit so that you can keep him rear facing for as long as possible. It is safer no matter how old they are! My personal preference is the Graco Extend2Fit but there are others available that rear face to 50lbs as well. The cervical image that I usually insert mid way through this had to be added as an image at the end because it’s too long when recommending a product, sorry!**
The best practice (from a safety standpoint) is to keep them rear facing until they max out the upper rear facing height or weight limit of their seat. Chances are they will max out one before the other, but once one limit is maxed out the seat is no longer safe rear facing. Rear facing is ALWAYS SAFER  no matter the age; it is even a safer position for adults if only cars were built with rear facing passenger seats. 
Until a child’s vertebrae have fully matured they are at risk of internal decapitation; this is when the spinal cord is severed inside the body, resulting in either paralysis or death. The maturation and ossification of the vertebrae is something that happens with AGE, not with SIZE, which is why extended rear facing (rear facing for as long as possible rather than just until age 2) is so important. Quoting from csftl.org:
“Toddlers are not just small adults; their bodies are still developing and those developmental changes make them more vulnerable than an adult. A toddler’s vertebrae are connected via cartilage rather than ossified bone. Those connections are called synchondroses, which are slowly closing over time. There are three major points of ossification, each with two synchondroses. According to a study published in the Association for the Advancement of Automotive Medicine, the first station to close is the C3, second is the axis, and third is the atlas. The results of the study show:
🔴 50% probability that the primary closure of the C3 is complete at age 2 and the secondary at age 3.
🟠 50% probability that the primary closure of the axis is complete at age 5 and the secondary at age 6.
🟡 50% probability that the primary closure of the atlas is complete at age 7 and the secondary at age 8 (Yoganandan, Pintar, Lew, Rao & Rangarajan, 2011).
This image to the right shows the cervical (top), thoracic (middle), and lumbar (bottom) vertebrae of a one year old (left, each photo) and six year old (right, each photo). Note the easily visible synchondroses in each. These findings show that before age two, none of the cartilaginous spaces have completed ossification. Those pieces of cartilage have the ability to stretch up to two inches. Yet only 1/4″ stretch is enough to rupture the spinal column, resulting in paralysis or death  (McCall, Fassett & Brockmeyer 2004).”
What this means is that a child’s vertebrae do not finish fully ossifying until ages 8-9 years old, no matter how “big for their age” they are. In fact, a child that is large for their age is actually at a higher risk forward facing than a smaller child of the same age because their head is a larger percentage of their overall weight and will exert more force on their neck in a forward facing collision. Statistics show that riders of ANY age (even adults) are safer riding rear facing. 
Many parents cite concerns of their child being “too cramped” in a rear facing car seat once their feet touch the seat back in front of them and their legs are required to bend when seated. Statistically, there are more leg and hip injuries reported in children who are FORWARD FACING because their legs swing up and hit the back of the front seats in a frontal collision. In contrast, in a rear facing frontal impact all of the momentum is absorbed into the back of the car seat rather than between their legs and the seat back in front of them. 
Kids with longer legs will find ways to sit comfortably that may not look comfortable to adults; whether by bending their knees, crossing their legs, stretching them up the seat back in front of them, hanging them over the sides of the car seat. Remember they are much more flexible than we are! The fact remains that rear facing is safer so long as the child fits within the maximum rear facing limits of the car seat. Even if a leg injury was to occur in an accident, I would ALWAYS rather a broken leg than a broken neck. Spacious leg room is not a necessity!
Every child will outgrow their car seat at a different rate than another. You should base your car seat choice on the growth trend of your specific child. Some kids are taller and skinnier, so ideally you would choose a seat with a higher rear facing height limit. Others are shorting and heavier, so a seat with a higher rear facing weight limit would be better. Both of my kids are mostly middle of the road on both height and weight. The goal should always be to rear face them for as long as possible. Once they have legitimately outgrown their seat (not just that they “look cramped”) they need to either be moved to a different seat with higher limits or switched to forward facing. For some higher percentile kids, that might really be 3 years old; for others, they might be able to remain rear facing until age 7 or 8. 
My oldest (boy) is 5 and is still rear facing comfortably in a Graco Extend2Fit (50lb rear facing limit.) Youngest (girl) is 3 and rear facing in a Graco 4Ever (40lb rear facing limit.)  My vehicle is a 2017 Ford Edge SUV.  Photos taken 5/27/21.

Answer

Check Safeintheseat on Instagram if you haven't already. She has a list(picture) of all the seat available in the US and Canada till 50lbs.
I have recommendations of European seats if needed.

Need to transition to a new carseat

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