Flexion in different positions. The New Ballard Scale is a scale that estimates the gestational age of a newborn infant based on physical and neuromuscular characteristics. Flexion in different positions is one of the six neuromuscular signs that are assessed using the scale. The other neuromuscular signs are square window, arm recoil, popliteal angle, scarf sign, and heel to ear.

The other statements are wrong because:

Skin thickness and presence of lanugo are physical signs, not neuromuscular signs. They are also assessed using the New Ballard Scale, along with plantar surface, breast, eye/ear, and genitals.

Creases on the bottom of feet are part of the plantar surface assessment, which is a physical sign, not a neuromuscular sign.

Scrotum development is part of the genital assessment, which is a physical sign, not a neuromuscular sign.

The New Ballard Scale can be used up to 4 days after birth, but usually within the first 24 hours. The scale is accurate only within plus or minus 2 weeks. The total score determines the gestational maturity in weeks.