Which of the following does not cross placenta:
A. Heparin
B. Morphine
C. Naloxone
D. Warfarin
Correct Answer: A. Heparin
Explanation
Question: Which of the following does not cross the placenta?
Correct answer: IgM (Immunoglobulin M)
Explanation:
The placenta acts as a selective barrier between the mother and the fetus. Some substances pass easily, while others do not.
- IgG:
✔️ Crosses the placenta
IgG antibodies are actively transported across the placenta. This is important because maternal IgG provides passive immunity to the fetus and protects the newborn during the first few months of life. - IgM:
❌ Does NOT cross the placenta
IgM molecules are large (pentameric structure), so they cannot pass through the placental membrane.
👉 Therefore, if IgM antibodies are found in the newborn, it usually indicates that the fetus has produced them due to intrauterine infection. - Other substances that cross the placenta (for understanding):
- Oxygen and carbon dioxide
- Nutrients (glucose, amino acids)
- Some drugs and toxins
- Certain viruses (e.g., rubella)
Key point to remember (Exam tip):
IgM does not cross the placenta; IgG does.
This question is very common in nursing, midwifery, and medical entrance exams.
Other Options Details
This question is related to placental drug transfer—which drugs can or cannot cross the placenta and affect the fetus.
Let’s describe each option clearly:
A. Heparin ✅ (Does NOT cross placenta)
- Heparin is a large, highly charged molecule.
- Because of its high molecular weight and polarity, it cannot cross the placental barrier.
- Safe anticoagulant in pregnancy.
- Commonly used to prevent or treat thromboembolic disorders in pregnant women.
👉 Key point: Heparin does NOT cross the placenta.
B. Morphine ❌ (Crosses placenta)
- Morphine is an opioid analgesic.
- It is lipid-soluble and low molecular weight, allowing it to cross the placenta easily.
- Can cause fetal respiratory depression and neonatal withdrawal if used near delivery.
👉 Key point: Morphine crosses the placenta.
C. Naloxone ❌ (Crosses placenta)
- Naloxone is an opioid antagonist.
- It can cross the placenta and may precipitate acute withdrawal symptoms in opioid-dependent fetuses.
- Used cautiously in pregnancy.
👉 Key point: Naloxone crosses the placenta.
D. Warfarin ❌ (Crosses placenta)
- Warfarin is an oral anticoagulant.
- It readily crosses the placenta.
- Causes warfarin embryopathy (nasal hypoplasia, stippled epiphyses) and fetal bleeding.
- Contraindicated in pregnancy.
👉 Key point: Warfarin crosses the placenta and is teratogenic.
✔ Correct Answer (Does NOT cross placenta):
➡️ A. Heparin
Summary Table
Drug Crosses Placenta? Effect Heparin ❌ No Safe in pregnancy Morphine ✔ Yes Fetal respiratory depression Naloxone ✔ Yes Fetal withdrawal Warfarin ✔ Yes Teratogenic
I hope that you liked this article.
Thanks!! 🙏 😊
Writer: Vandita Singh, Lucknow (GS India Nursing Group