Phenylephrine causes all of the blood vessels in your body to squeeze tighter. This helps lower the uncomfortable swelling behind your nose and lets you breathe easier. While your clogged sinuses love this effect, your baby doesn’t.
When phenylephrine makes all your blood vessels constrict, that includes the ones feeding the uterus and umbilical cord, which can leave your baby with less-than-ideal oxygen levels for a long time. Most beefy over-the-counter cold medications have phenylephrine mixed in, so if your sinuses are begging for relief, you can ask your healthcare provider about other decongestant options that are safer for the baby.