The anomaly scan, also sometimes called the anatomy scan, 20-week ultrasound, or level 2 ultrasound, evaluates anatomic structures of the

fetus A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic development, embryonic stage, the fetal stage of development takes place. Pren ...

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

, and maternal pelvic organs. This scan is an important and common component of routine

prenatal care Prenatal care, also known as antenatal care, is a type of preventive healthcare for pregnant individuals. It is provided in the form of medical checkups and healthy lifestyle recommendations for the pregnant person. Antenatal care also consists of ...

. The function of the

ultrasound Ultrasound is sound with frequency, frequencies greater than 20 Hertz, kilohertz. This frequency is the approximate upper audible hearing range, limit of human hearing in healthy young adults. The physical principles of acoustic waves apply ...

is to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy, to assess for congenital malformations and multiple pregnancies, and to plan method of delivery.

Procedure

This scan is conducted between 18 and 22 weeks'

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregn ...

, but most often performed at 19 weeks, as a component of routine

. Prior to 18 weeks' gestation, the fetal organs may be of insufficient size and development to allow for ultrasound evaluation. Scans performed beyond 22 weeks' gestation may limit the ability to seek pregnancy termination, depending on local legislation.

Two-dimensional A two-dimensional space is a mathematical space with two dimensions, meaning points have two degrees of freedom: their locations can be locally described with two coordinates or they can move in two independent directions. Common two-dimension ...

(2D) is used to evaluate fetal structures,

, and

amniotic fluid The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products betwee ...

volume. Maternal pelvic organs are also evaluated. Views are obtained using an

abdominal ultrasound Abdominal ultrasonography (also called abdominal ultrasound imaging or abdominal sonography) is a form of medical ultrasonography (medicine, medical application of ultrasound technology) to visualise abdomen, abdominal anatomy, anatomical structu ...

probe, but a vaginal ultrasound probe may also be used to evaluate for placenta previa and cervical length.

Three-dimensional In geometry, a three-dimensional space (3D space, 3-space or, rarely, tri-dimensional space) is a mathematical space in which three values (''coordinates'') are required to determine the position (geometry), position of a point (geometry), poi ...

(3D) ultrasound is not recommended for routine use during anomaly scan, but 3D ultrasound may be used to further evaluate suspected abnormalities in specific fetal features.

Components

Human placenta umbilical cord Ultrasound by Dr

A standard anatomy scan typically includes: * Fetal number, including number of amnionic sacs and

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It is also present around the embryo of other animals, like insects and molluscs. Structure In humans and other therian mammals, the cho ...

ic sacs for multiple gestations * Fetal cardiac activity * Fetal position relative to the

uterus The uterus (from Latin ''uterus'', : uteri or uteruses) or womb () is the hollow organ, organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic development, embryonic and prenatal development, f ...

and

cervix The cervix (: cervices) or cervix uteri is a dynamic fibromuscular sexual organ of the female reproductive system that connects the vagina with the uterine cavity. The human female cervix has been documented anatomically since at least the time ...

* Location and appearance of the

, including site of

umbilical cord In Placentalia, placental mammals, the umbilical cord (also called the navel string, birth cord or ''funiculus umbilicalis'') is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord i ...

insertion * Amnionic fluid volume * Gestational age assessment * Fetal weight estimation * Fetal anatomical survey * Evaluation of the maternal uterus, tubes,

ovaries The ovary () is a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are endocr ...

, and

bladder The bladder () is a hollow organ in humans and other vertebrates that stores urine from the kidneys. In placental mammals, urine enters the bladder via the ureters and exits via the urethra during urination. In humans, the bladder is a distens ...

when appropriate

Medical uses

Second-trimester

screening for aneuploidies, such as Edwards syndrome and Patau syndrome, is based on looking for soft markers and some predefined structural abnormalities. Soft markers are variations from normal anatomy, which are more common in aneuploid fetuses compared to euploid ones. These markers are often not clinically significant and do not cause adverse pregnancy outcomes, but used in order to decide whether additional tests, which will be more accurate, are needed. Placental evaluation is vital for planning method of delivery, as attempting natural childbirth can cause fatal amounts of

blood loss Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural opening such as the mouth, nose, ear, urethra, v ...

in the mother if the placenta is covering the cervix ( placenta praevia) and significantly increases the risk of

stillbirth Stillbirth is typically defined as fetus, fetal death at or after 20 or 28 weeks of pregnancy, depending on the source. It results in a baby born without vital signs, signs of life. A stillbirth can often result in the feeling of guilt (emotio ...

. Additionally, placental and

abnormalities are also associated with certain fetal genetic abnormalities.

Sex determination

Fetal sex is often noted during the anomaly scan, but can also be detected during the quad scan. However, performing an ultrasound for the sole purpose of determining fetal sex without a medical indication is not recommended.

Limitations

Although ultrasound is a useful screening tool for fetal anomalies, the sensitivity of this scan is highly variable and potentially detects only 40% of all fetal anomalies. The ability of this scan to detect structural fetal abnormalities depends on a variety of factors. These factors include the severity of the anomaly, the background risk of the study population, gestational age at time of scan, the expertise of the ultrasound technician and interpreting

obstetrician Obstetrics is the field of study concentrated on pregnancy, childbirth and the postpartum period. As a medical specialty, obstetrics is combined with gynecology under the discipline known as obstetrics and gynecology (OB/GYN), which is a surgi ...

, and maternal

obesity Obesity is a medical condition, considered by multiple organizations to be a disease, in which excess Adipose tissue, body fat has accumulated to such an extent that it can potentially have negative effects on health. People are classifi ...

References