When is PUBS performed, with the greatest miscarriage risk?

• A. 12-16 weeks
• B. 15-20 weeks
• C. 21-26 weeks
• D. 18-22 weeks

Answer: (D)

Percutaneous umbilical blood sampling (PUBS) is an invasive, ultrasound-guided procedure that accesses fetal blood via the umbilical vein to obtain diagnostic information. Because it involves penetrating the maternal abdomen and the fetal vessels, there is a real risk of fetal loss, and this risk varies with gestational age.

The highest miscarriage risk for PUBS occurs in the mid-second trimester, around 18–22 weeks. At this stage the umbilical vessels are large enough to sample reliably and the placenta is accessible, making the procedure technically feasible, but the fetus is still vulnerable to procedure-related trauma. Earlier windows (12–16 weeks or 15–20 weeks) are generally not the standard window for PUBS due to greater technical challenges and different risk profiles, while later windows (21–26 weeks) are more limited by fetal size, fetal position, and other complications, so the reported miscarriage risk is not as high as in the 18–22 weeks window.