According to research, the rate of mental retardation is generally considered higher among twins compared to singletons, primarily due to the increased risk of lower birth weight and shorter gestational age associated with twin pregnancies, which can impact neurodevelopment.

The incidence of Twins in cerebral Palsy and Mental Retardation
Rosanoff and Inman- Kane (1933) studied 234 pairs of mentally defective twins. They found a higher incidence of mental defects in twins than in singletons. McKinney, Dunphy and Pessin (1958) and Drillien (1959) made similar observations.

Research indicates that premature births (especially very preterm or extremely preterm) are associated with a higher risk of developmental delays, including potential cognitive impairments and intellectual disabilities.    

Brain development: The brain undergoes significant development during the final weeks of pregnancy. Premature infants miss part of this critical period, which can affect brain growth and function.
Medical complications: Premature infants often face medical issues such as respiratory problems, infections, or intraventricular hemorrhage, which can negatively impact cognitive development.

AI:  Yes, the brain undergoes significant development during the final weeks of pregnancy, especially during the third trimester:
Brain volume: The brain's volume increases 20-fold between 20 weeks and 38–40 weeks of gestation.
Brain size: The brain's size increases rapidly during the last trimester.
Brain folding: The brain's outer layer, or cortex, folds during the last trimester.
Neuron connections: Connections between neurons develop during the last trimester.
White matter: Major long-range signaling pathways made of white matter form and establish themselves by birth.
The brain's development continues after birth, with the average baby's brain doubling in size during the first year. By age 3, the brain is about 80% of its adult size, and by age 5, it's about 90%.

A more Omnivorous diet, may create omnivorous characteristics such as a shorter gestation, more twin births and smaller brain size.  The following links support this.

The mystery over why human brains have shrunk over time
The brains of modern humans are around 13% smaller than those of Homo sapiens who lived 100,000 years ago.  Exactly why is still puzzling researchers.

Nearly 1 in 10 U.S. children have been diagnosed with a developmental disability, CDC reports

Preterm birth rate in the US is rising, study finds, but the reasons are a mystery

The number of twins in the United States has been increasing, especially since the 1980s:
1980s–2014
The number of twin births increased by 79% from 1980 to 2014. The twin birth rate increased from 18.9 to 33.2 per 1,000 births.
2020–2021
The number of twin births increased by 2% from 2020 to 2021, from 112,437 to 114,161.
Overall trends
The twin birth rate has increased across all age and race groups, and in all U.S. states. The rate of increase averaged more than 2 percent a year from 1980 through 2004

https://www.cdc.gov/nchs/data/databriefs/db80.pdf

https://www.cdc.gov/nchs/products/databriefs/db351.htm#:~:text=Following%20years%20of%20relative%20stability,of%20residence%20for%202014%E2%80%932018.