Third trimester starting this week

Whichever way one calculates it (dividing into thirds of the gestation time, the last menstrual period date, or going by developmental hallmarks), the third trimester is coming up this week!

I'm so excited to meet this baby! I hope that he or she continues to be a for-internal-use-only baby for another trimester, though.

At 27 weeks, the baby currently weighs a bit over 2 lbs, enough that he or she is on this textbook graphic. Birth weight is THE classic example of stabilizing selection that I use when I'm teaching evolution. Stabilizing selection means that there's a happy medium, in this case of baby weight at birth. Smaller babies are less likely to make it due to prematurity and not being able to regulate their body temperature, much larger babies are likely to have more difficulty in making an exit.



Note, though, that this (very scary!) graph is based on births from 1935 to 1946. The advent of caesarian birth becoming more available has changed this (it's now less dangerous to be a big baby), and technology has also greatly increased the survival of smaller, premature infants (also less dangerous to be a small baby than it used to be).

At nearly 27 weeks, if the baby were born at this point, s/he would have a pretty good chance of survival with the help of modern medicine. See March of Dimes for more information. I'm profoundly grateful for the fact that situation for preemies has improved so much over 70 years.

However, here's hoping to another 10 to 14 weeks of being a pregnancy blog rather than a baby blog, during which the baby will get bigger, put on important body-regulating baby fat, and develop lungs and eyes more fully!