26 weeks pregnant, or 24 weeks after conception: surfactant production

Your baby is starting to get a bit chubby and the fat reserves, developed throughout their growth, now account for almost 3% of their total weight. At nearly 820 grams, your baby has reached a third of its final weight and is starting to run out of room. The nerves are completing their development, and the brain is now showing an increasing number of convolutions. 

The lungs begin to produce surfactant, the substance that allows the lung alveoli to inflate at the time of birth and optimise the newborn's breathing.