Definition of First Trimester Pregnancy
The first trimester of pregnancy is defined as a gestational age (GA) of 13 weeks 6 days or less. 1
Key Components of the Definition
Gestational Age Calculation
- Gestational age (GA) represents the duration of pregnancy and is reported as the number of weeks and number of days 1
- GA is calculated using the best obstetric estimate, typically based on:
- The first day of the last menstrual period (LMP), and/or
- Early dating ultrasound examination 1
- Alternate terms for GA include menstrual age, gestational duration, clinical age, or clinical dates 1
Special Considerations for Assisted Reproduction
- In the setting of assisted reproductive technologies, pregnancy is dated based on clinical factors such as:
- Time of intrauterine insemination
- Embryo transfer timing 1
Important Terminology Guidance
Appropriate Use of "Normal"
- The term "normal" should only be used to describe first-trimester ultrasound examination characteristics (such as "normal first-trimester US examination" or "normal location") 1
- Avoid the phrase "normal first-trimester pregnancy" as fetal anomalies may be undetectable at this early gestational age 1
Clinical Context
- The first trimester represents a critical window of fetal development 2
- This period encompasses early pregnancy ultrasound examinations performed for clinical indications including:
- Confirmation of pregnancy
- Dating
- Presence of cardiac activity
- Determining pregnancy location and number
- Evaluation of vaginal bleeding or pain 1
Standardized Reporting
This definition comes from the Society of Radiologists in Ultrasound consensus conference recommendations, which established standardized terminology to ensure clear, scientifically based, and medically appropriate communication that minimizes bias and harm in first-trimester pregnancy reporting 1