Placental Insulin Resistance
By the third trimester, placental hormones (like Human Placental Lactogen) induce massive insulin resistance. Total Daily Dose (TDD) can increase by 200-300%. This breaks the underlying assumptions of many commercial algorithms.
| Trimester | Physiological Change | Algorithm Adjustment Required |
|---|---|---|
| First | Increased insulin sensitivity | Risk of severe lows. Requires weaker basal profiles. |
| Second | Steep resistance begins | Constant re-evaluation of Carb Ratio and ISF. |
| Third | Peak resistance | Max out pump basal rates; U-200 insulin may be required. |
Algorithm Target Limitations
The standard of care for pregnancy is an HbA1c < 6.0% and fasting glucose < 90 mg/dL. Most commercial systems (like Control-IQ) hard-code their target to 110-112 mg/dL and cannot be lowered. Therefore, many users switch to DIY Looping or CamAPS FX (which allows targets down to 80 mg/dL) during pregnancy to achieve these strict physiological goals.
Hardware Constraints (U-200 Insulin)
Because resistance is so high, a 300U pump reservoir might only last 1.5 days. To avoid constant site changes, clinicians sometimes prescribe U-200 Humalog (twice as concentrated). If put into a pump, every setting in the pump must be divided by 2 to prevent fatal overdoses. Use our calculators to double-check math.
FAQ
- Is CGM accuracy affected by pregnancy?
- Physiological changes in interstitial fluid can cause CGM lag to increase slightly, but they remain highly accurate. Fingerstick validation is still heavily recommended.
- What happens immediately post-partum?
- Upon delivery of the placenta, insulin resistance vanishes instantly. Pump settings must be immediately reverted to pre-pregnancy levels to avoid severe hypoglycemia. Have Nightscout profiles ready to switch.