Terminology
- Preterm: a birth that occurs before 37 completed weeks of gestation
- Term: a birth between 37 weeks and 42 weeks of gestation
- Post term: a birth that occurs after 42 weeks of gestation
- 'Normal' weight 2.5kg-4.0kg
- Over 4.0kg - large for gestational age (LGA)
- Under 2.5kg - small for gestational age (SGA)
Events in the 3rd trimester
- Daily weight gain of 24g during 3rd trimester
- Approx. 7g of fat per day in last 4 weeks
- Transplacental transfer - iron, vitamins, calcium, phosphate and antibodies
Surfactant
- Surface tension is the attraction of the molecules in a liquid to each other, pulling them together and minimising surface area
- Alveoli are the small, fluid-lined sacs where gas collects and diffuses into the blood during inhalation
- The molecules of this fluid pull together due to surface tension, in turn pulling the walls of the alveoli towards each-other, attempting to collapse the space in the alveoli
- Surfactant is a fluid produced by type II alveolar cells; it keeps the alveoli inflated and maximises the surface area of the alveoli by reducing the surface tension of the fluid in the lungs
- Essentially provides a barrier that reduces the water molecules tendency to pull towards each other
- The result is that surfactant keeps the alveoli inflated and maximises the surface area of the alveoli - increases lung compliance
- Surfactant also promotes equal expansion of all alveoli during inspiration
- As an alveolus expands, the surfactant becomes more thinly spread and therefore the surface tension increases, making it more difficult to expand that alveolus further
- This stops one alveolus expanding massively whilst another alveolus only expands a little
- Type II alveolar cells become mature enough to start producing surfactant between 24 and 34 weeks gestation - pre-term babies have problems associated with reduced pulmonary surfactant.
Challenges for baby during labour
- During contractions, the baby is in a hypoxic environment
- Foetal Hb helps release O2 to help with this
- Prolonged labour reduces fetal reserves