A heterotopic pregnancy involves multiple embryos implanted in different locations, usually one in the uterus and another ectopically. This rare condition can pose significant risks to both the mother and the developing embryos. Heterotopic pregnancy is very rare. An interstitial heterotopic pregnancy is even rarer, representing 2%–4% of all ectopic pregnancies, but it has a much higher mortality rate overall because of the complexities involved in diagnosis and management. This case involves a rare instance of interstitial heterotopic pregnancy, likely resulting from assisted reproductive technology. Ultrasound revealed a live intrauterine pregnancy and an interstitial ectopic pregnancy sac. A 40-year-old woman, G5P1Ab1Ep2, was referred to the hospital triage in her 7th week of pregnancy due to abnormal ultrasound findings. Considering the clinical conditions and the possibility of uterine rupture with increasing gestational age, a decision was made to reduce the ectopic pregnancy by injecting KCl. Ultimately, a multidisciplinary approach involving obstetricians, gynecologists, and, when necessary, maternal–fetal medicine specialists is vital to optimize outcomes for both the mother and the developing fetus.