Intrauterine gestation is a normal finding on a first-trimester ultrasound and means that the pregnancy is taking place inside of the uterus. This is in contrast to ectopic pregnancy that can occur inside of a fallopian tube, ovary, cervix or the abdomen, according to the Mayo Clinic.
After an egg is released from an ovary, it is caught by large projections from the fallopian tubes called fimbriae and propelled toward the uterus. Fertilization takes place in the fallopian tubes. Within 24 hours, a sperm cell travels in the opposite direction of the egg, from the vagina, into the cervix, through the uterus and into the fallopian tube to meet it and penetrate it, forming a zygote, according to the Merck Manual.
Hair-like projections in the fallopian tubes propel the zygote towards the uterus on a three- to five-day trip that ends with implantation. The zygote is now called a blastocyst because it contains multiple layers that will later form into an embryo, placenta and other membranes. Implantation is complete on day nine or 10, forming an intrauterine gestation, according to the Merck Manual.
One of the primary indications for an ultrasound in the first trimester is to make sure that this fertilization process occurred correctly and that intrauterine gestation has taken place, as well as to rule out any suspicion of ectopic pregnancy, according to the International Society of Ultrasound in Obstetrics and Gynecology.