“When people, including physicians, talk about "brain waves
" and "brain activity" they are referring to organized activity in the cortex.
While no embryo or fetus has ever been found to have "brain waves," extensive EEG studies have been done on premature babies.
A very good summary of their findings can be found in Pain and its effects in the human neonate and fetus," a review article (often cited by "pro-lifers" writing about fetal pain, but not about brain development) by K.J.S. Anand, a leading researcher on pain in newborns, and P.R. Hickey, published in NEJM:
Functional maturity of the cerebral cortex is suggested by fetal and neonatal electroencephalographic patterns...
First, intermittent electroencephalograpic bursts in both cerebral hemispheres are first seen at 20 weeks
gestation; they become sustained at 22 weeks and bilaterally synchronous at 26 to 27 weeks.
There are reasons, based on the physics of the EEG, why this has to be so.
Remember, an EEG involves measuring varying electrical potential across a dipole, or separated charges. To get scalp or surface potentials from the cortex requires three things: neurons, dendrites, and axons, with synapses between them.
Since these requirements are not present in the human cortex before 20-24 weeks
of gestation, it is not possible to record "brain waves" prior to 20-24 weeks. Period. End of story.
Scientists do not attempt to find electrocortical activity in embryos
and fetuses because they know more about the physical structure of the developing human brain
than they did in 1963. “