Baby's Development - Third Trimester

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Baby's Development - Third Trimester

By the beginning of the third trimester, the baby’s vital organs and skeleton are complete. The final trimester of the pregnancy involves the maturing and strengthening of these systems.

The amniotic fluid around the baby increases during the final trimester of pregnancy, and by term, has almost doubled in volume from what it was at 20 weeks. The baby is continuously swallowing the fluid, which processed by its kidneys and passed back out into the amniotic fluid. Waste products cross the placenta and enter the mother’s blood stream. The water content of the fluid changes approximately every 3 hours!

The fine bronchial structures of the lungs continually subdivide from around 26 weeks onwards. Until the baby is born, the air spaces in the baby’s lungs are filled with liquid which increases throughout the third trimester, expanding the space available in the baby’s lungs. The fluid also plays a role in the production of surfactants which are produced in increasing amounts from 32 weeks, and coat the delicate lung structures and prevent them from sticking together. This is a key stage of the baby’s lung development and is generally complete at around 35 weeks.

The breathing movements that the baby has been making become integrated with the baby’s circadian rhythms including heart rate, temperature, body movements and sleep.

The breathing movements made by the baby actually stimulate the growth of the lungs’ tissues and muscles. They may even increase the amount of blood being pumped around the baby’s body, sending it to vital organs like the baby’s brain and placenta.

Towards the end of pregnancy small antibodies from the mother pass across the placenta to the baby. These antibodies will confer passive immunity to the baby from everything which the mother is immune to. This will last for the first 3 months and will be supported by further antibodies in the baby’s milk, if the baby is breastfed.

Exactly how labour is initiated is unclear and may involve several chemical pathways, but researchers at the University of Texas Southwestern Medical Center in Dallas think that they may have an answer, based on their work with mice. It appears that the surfactant which is secreted by the baby’s developing lungs in the final weeks of pregnancy, ultimately acts as a hormone and sends a neuro-chemical message to the mother’s uterus that the baby’s lungs are sufficiently mature to handle a life of breathing air.

As the baby "breathes" the amniotic fluid, the surfactant is released into the fluid and it binds to immune cells from the baby already in the amniotic fluid and activates them. These activated immune cells make their way to the wall of the uterus and embed themselves there.  They then produce a chemical which stimulates an inflammatory response in the uterus and this ultimately triggers labour.

Condon, J.C., Jevasuria, P., Faust, J.M., Mendelson, C.R. (2008). Surfactant protein secreted by the maturing mouse fetal lung acts as a hormone that signals the initiation of parturition. Proceedings of the National Academy of Sciences. Vol. 101 no. 14. 4978–4983, doi: 10.1073/pnas.0401124101. Available online at [Accessed on 24 March 2014]