Maternal metabolism across pregnancy
Pregnancy has two broad metabolic phases: a mainly anabolic first half that stores fuel, and a mainly catabolic second half that releases it, with the mother less sensitive to insulin.
Remember
Early pregnancy (the first half) is mainly anabolic and stores fuel; late pregnancy (the second half) is mainly catabolic with insulin resistance (less sensitivity to insulin), which keeps glucose and fatty acids available for the foetus. These are broad patterns, and individual women vary.
Memory aid
build first, share later
Stores are laid down in the first half of pregnancy and shared with the foetus in the second half.
During pregnancy the mother's metabolism must meet the needs of the growing foetus, and it changes in two phases.
In the first half, metabolism is anabolic, which means the mother builds and stores fuel. Her fat and nutrient stores increase, ready to supply the foetus and to breastfeed after birth.
In the second half, metabolism is catabolic, which means fuel is released from stores rather than laid down. The mother also becomes less sensitive to insulin, which is called insulin resistance. As a result, glucose and fatty acids stay in her blood for the foetus to use.
These changes are driven by the foetoplacental unit, a system made up of the foetal adrenal glands, the foetal liver and the placenta. It produces hormones that alter the mother's insulin sensitivity. In the second half, the mother's blood glucose tends to be higher after meals because insulin works less well, but it can fall overnight because her glycogen stores are used up.
This is a normal adaptation that favours the foetus. Most women make extra insulin and stay in the normal range, but if they cannot compensate, gestational diabetes can result.
Key facts
- First half
- anabolic, with the mother's fat and nutrient stores increasing.
- Second half
- catabolic, with fuel released from stores and decreased insulin sensitivity.
- Driver
- hormones from the foetoplacental unit alter the mother's insulin sensitivity.
- After meals
- in the second half, blood glucose tends to be higher, because insulin works less well.
- Overnight
- in the second half, blood glucose can fall, because her glycogen stores are used up.
- Extra insulin
- most women make extra insulin and stay in the normal range; if they cannot, gestational diabetes can result.
Exam traps
- Increased insulin sensitivity, which pushes glucose into the mother's own tissues, is not the pattern of the second half of pregnancy, because it would leave less glucose for the foetus and fits the first half better.
- Sustained high glucose when fasting is not the normal pattern in later pregnancy, because the mother's glycogen stores are used up overnight and she tends to have low glucose when fasting.
- Maternal metabolism does not stay anabolic throughout pregnancy, because it is anabolic in the first half and catabolic in the second.
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