[viridian]

I think your grapefruit comparison is inaccurate unless you are referring to just the head. The Mayo Clinic's guide to pregnancy lists a baby's head at 24 weeks, just the head alone, at 8.7 inches (circ), but total length heel to crown is also 12.2 inches, which is about 60% the average length of a full term baby.

23 weeks is also more or less the borderline for where we have standardized national practices for saving an extreme premature delivery. I've read studies that put viability in the NICU at week 23 anywhere from 10% on the extreme low end, to 50% on the extreme high. By week 24 that jumps to ~30-75%.