Pregnancies occupying the rudimentary horns of anomalous uteri are uncommon forms of ectopic gestation and occur in 1 in 76,000 pregnancies. It occurs through the trans-peritoneal migration of the spermatozoon or the trans-peritoneal migration of the fertilized ovum. 70–90% rupture before 20 weeks and can be catastrophic. It is very rare to reach a full term for a rudimentary horn pregnancy, though some literature shows 10% pregnancy reaches full term and fetal survival rate was 2%. Despite advances in ultrasound and other diagnostic modalities, prenatal diagnosis remains elusive, with confirmatory diagnosis being laparotomy. The diagnosis can be missed in ultrasound especially in inexperienced hands. Here we report a full term rudimentary horn pregnancy in 20 years primigravida lady which was retained for about a month after rupture and presented with peritonitis and anemia. The patient was managed with emergency laparotomy and the ruptured and infected rudimentary horn excised with its tube and ovary. The woman survived and discharged in a good condition. There is a need for an increased awareness of this condition especially in developing countries among health professionals. Especially in a spontaneous uterine rupture in primigravida as in our case rudimentary horn pregnancy should be suspected.