Ultrasound Evaluation of Pregnancy in the First Trimester

The first trimester of pregnancy starts with the first day of the last menstrual period (LMP) and continues until the end of the twelfth week. Although, conception normally occurs two weeks after the LMP, the LMP is chosen as the start of the first trimester because it is a recognizable sign for most women. The term menstrual age and gestational age are used interchangeably, but should not be confused with embryologic age which begins with conception and is therefore 2 weeks less than menstrual and gestational age. The first trimester can be divided into four stages: ovulation, conceptus, embryonic, and fetal. The ovulation stage occurs during the first two weeks after the LMP. This stage is followed by the conceptus stage which takes place between the third and the fifth week. The conceptus stage starts with fertilization which normally occurs within one day of ovulation, or day 15 of a 28 day menstrual cycle. It is during the conceptus stage that implantation of the blastocyst into the endometrium occurs. This may result in vaginal bleeding that can be confused with an irregular menstrual cycle. By the end of the fourth week a measurable amount of human chorionic gonadotropin (hCG) is produced, resulting in a positive pregnancy test. At this point the products of conception measure between 2 to 3 Abstract