Final answer:
Amniotic fluid can become foul and yellow if the fetus has passed meconium, which is the baby's first bowel movement. This can occur during childbirth when the amniotic sac ruptures. Meconium can interfere with the antibiotic properties of the amniotic fluid and cause irritation to the fetus's skin.
Step-by-step explanation:
Amniotic fluid is normally clear and watery, but it can become stained greenish or yellowish if the fetus has passed meconium, which is the baby's first bowel movement. This typically happens during childbirth when the amniotic sac ruptures. Meconium is a sterile substance but it can interfere with the antibiotic properties of the amniotic fluid, making the newborn and mother more vulnerable to bacterial infections. It can also irritate the fetus's skin.