ULTRASOUND OF THE EARLY FIRST TRIMESTER

Dr.kobra ShojaeiFellowship of perinatology

The progression of transvaginal sonographic findings in normal early first trimester pregnancies follows a highly predictable pattern, with a gestational age variability of approximately ±0.5 week: gestational sac at 5.0 weeks, yolk sac at 5.5 weeks, embryowith heartbeat at 6.0 weeks, and amnion at 7.0 weeks.

In a woman with a positive pregnancy test and no evidence of an intrauterine or ectopic pregnancy on ultrasound (pregnancy of unknown location [PUL]), a single hCG measurement does not reliably distinguish a normal intrauterine pregnancy from a failed intrauterine pregnancy or an ectopic pregnancy.

In a woman with a positive pregnancy test, any round or oval intrauterine fluid collection should be interpreted as highly likely to be a gestational sac, not a pseudogestational sac or decidual cyst, and treatments that could damage an intrauterine pregnancy should be avoided.

An extraovarian mass can be distinguished from an intraovarian lesion, such as a corpus luteum, by observing its motion relative to the ovary when pressure is applied by the transvaginal transducer.

Sonographic findings definitive for failed intrauterine pregnancy include crown-rump length (CRL) of at least 7 mm without cardiac activity, mean sac diameter (MSD) of at least 25 mm without an embryo, and no embryo with a heartbeat on afollow-up scan at specific time intervals after the initial scan.

Sonographic findings suspicious, but not definitive, for failed intrauterine pregnancy include CRL of less than 7 mm without cardiac activity, MSD of 16 to 24 mm without an embryo, no embryo with a heartbeat 6 weeks or more after the first day of the last menstrual period (LMP), empty amnion sign, expanded amnion sign, and a large yolk sac.

Risk factors for pregnancy failure when embryonic heartbeat is seen include slow heart rate, large subchorionic hematoma, and small gestational sac size.

Pregnancy number (singleton, twin, triplet, and higher order multiples) assessed prior to 6 weeks of gestation may subsequently decrease owing to the vanishing twin phenomenon or may increase by one or more additional embryos that appear on a follow-up scan.

NORMAL TRANSVAGINAL SONOGRAPHICFINDINGS IN THE EARLY FIRST TRIMESTER

Approximately 1 week after fertilization, the pregnancy implants in thedecidua on one side of the uterine cavity ,but it is not yet visible on ultrasound for about 2 more weeks.

The gestational sac is first seen on transvaginal sonography at 5.0 weeks of gestation as a fluid collection 2 to 3 mm in diameter normally located in the central echogenic region of the uterus, which corresponds to the decidua .It grows at the rate of 1 mm/day over the course of the next week.

The first structure that is identifiable within the gestational sac is the yolk sac, which appears as a circular structure up to 6 mm in diameter

It can be seen beginning at 5.5 weeks and remains visible for most of the first trimester.

The embryo is first visible at 6.0 weeks, initially appearing as a 1- to 4-mm echogenic structure adjacent to the yolk sac with a flickering motion inside it, representing the beating Heart .

By 7.0 weeks, the embryo is approximately 1 cm in lengt h9-11 and is still fairly featureless. At that point, the amnion first becomes visible around the embryo, as the amniotic cavity enlarges with fluid between the embryo and the amnion

By 8.0 weeks’ gestation, the embryo is 16 mm in length, and individual body parts can be identified on ultrasound. The embryonic head can be distinguished from the trunk, and limb buds are visible.