From my fostering experience, I would echo what both previous replies have said. I have often noted that younger children in sibling groups have more, or at least different struggles to their older siblings. The individual child's personality, strengths, etc. is definitely a factor, but, importantly, it can be the case that a struggling parent manages relatively well with one child, and that child might receive a few years of good enough parenting before subsequent children come along. By the time the third or fourth child comes along, the family situation can have deteriorated, the mother may experience considerably more stress while pregnant, resulting in increased foetal stress. Severe family challenges may mean that the younger children are born into chaos from day one. I think changes to the family set up can also be a factor. Perhaps the first child was born into a relatively stable situation, but then by the time the next child came along, domestic violence was a feature in the family, or a mental health crisis, or a financial crisis. So, even biological siblings can have very different experiences both in utero and during early life.
I also think that the impact of very early removal is not discussed enough, and not enough is known about it. I'm not saying it's worse than being removed later, but it's different and I don't think we know enough about the very primal instincts of very young infants and the effects on them when they are dislocated from the person whose voice, smell, heartbeat etc. they have become familiar with in the womb. I have fostered infants straight from the hospital a few days after birth and noted that they often pull away, don't respond well to comfort, cuddles etc. A psychologist friend of mine told me that newborns are primed to respond to their mother's smell. I wonder if they feel completely cast adrift when that smell disappears and is replaced by nothing familiar. There must be a very primal fear response. When I was fostering babies, I always noted how my friend's babies seemed to 'nestle' into their bodies, whereas all my foster babies seemed to hold themselves a little unnaturally when I carried them or rocked them - almost stiff and resistant to relaxing against me like other babies relax against their mums. I found it very sad.