“It’s not a contraindication until someone checks.”

The maternal–fetal drug pathway

The fetus usually does not receive medications directly from the mother’s bloodstream. Instead, most drugs must cross the placenta, which acts as a selective barrier between the maternal and fetal circulations. Similar to the blood-brain barrier, the placenta allows some substances to pass while restricting others, although many medications can still cross depending on their size, lipid solubility, protein binding, and other pharmacologic properties.

Maternal blood and fetal blood normally do NOT directly mix.

Instead, exchange happens across layers of placental tissue:

• Syncytiotrophoblast
  • Outer layer directly exposed to maternal blood
  • Major site where drugs cross
• Cytotrophoblast
  • Inner cellular layer (more prominent early in pregnancy)
• Basement membrane + fetal capillary lining
  • Final layers before reaching fetal circulation

To reiterate, because the placenta is a selective barrier, not a complete filter, allowing many drugs to pass based on factors such as molecular size , lipid solubility, protein binding, and ionization. Small, fat-soluble, and less protein-bound medications cross more easily, meaning the fetus may be exposed to medications given to the mother.

A good example of this would be Fentanyl. It’s a small, lipid-soluble medication that easily crosses the placental barrier.

As pregnancy progresses, the placenta becomes thinner, making drug transfer easier. This is why the protocols reference caution or contradiction around late term (third trimester) administration.

So what drugs are safe?

That’s a loaded question and it depends who you ask. There was once a time when smoking during pregnancy was considered fine, luckily medicine is always changing.

Consultation with medical control prior to pain management in the third trimester is strongly suggested.

NSAIDs such as Ibuprofen and Ketorolac are contraindicated in pregnancy. They can cause rare and serious kidney problems in the unborn baby, leading to low amniotic fluid.

“In situations where there is a direct threat to the life of the patient, it is often necessary to administer a higher risk medication to preserve the life of the mother, regardless of the risk to the unborn child.” – Nancy Caroline

What medicines should you give in pregnancy?

If they’re seizing secondary to eclampsia you should definitely treat them with Midazolam first (10 mg IM/IN, 5 mg IV) followed by 4 grams of Magnesium IV over 20 minutes.

Eclampsia is a progression of pre-eclampsia which presents itself as hypertension, headache, confusion, hyperreflexias. Patients who are greater than 20 weeks pregnant or who have given birth within 6 weeks are at highest risk.

TL;DR

The safest drug in pregnancy is the one that keeps Mom alive.