24 February 2021 Author : Pedro Oliveira Pregnancy surveillance includes most often 3 ultrasounds , in the first, second and third trimesters of pregnancy. Sometimes a first ultrasound scan is also performed earlier. Currently, the available technology also allows for 3D-4D ultrasounds, although these are not part of routine pregnancy surveillance. The general objectives of ultrasounds performed during pregnancy are to confirm: Fetal viability; Date the pregnancy correctly; Complement the screening for possible fetal malformations; Assess fetal development and well-being; Estimate the weight of the fetus. Early obstetric ultrasound Early obstetric ultrasound is performed before 11 weeks of pregnancy. Its objectives are: Exclude an extrauterine pregnancy; Confirm pregnancy and gestational age; Check the number of fetuses present; Check the viability of the fetuses present; Check heart rate. This ultrasound is usually performed through the abdomen. In some cases where it is done earlier, it may be necessary to perform through the vagina. Ultrasound of the first trimester or screening for fetal anomalies The screening for fetal anomalies ultrasound is performed at the end of the first trimester of pregnancy, between 11 and 14 weeks . This ultrasound scan is performed through the abdomen and its main objectives are: Determine the gestational age; Screen for fetal abnormalities. Gestational age can be determined from several measures, such as: The cranio-caudal length; The bi-parietal diameter; The head circumference; The length of the femur. For screening for fetal anomalies, the translucency of the neck and nasal bones is evaluated , whose deviations from normal values may be associated with an increased risk of: Chromosomal disorders; Heart diseases; Some genetic diseases. The ultrasound of the first trimester makes it possible to estimate the absence of chromosomal disorders with a certainty of about 70%. The degree of certainty increases to more than 90% when associated to: Maternal age; Performing a biochemical screening of the first trimester. Ultrasound of the second trimester or fetal morphological ultrasound Fetal morphological ultrasound is performed between 18 and 24 weeks of pregnancy and through the abdomen. In many cases it is the last ultrasound scan taken during pregnancy. The 18-24 week ultrasound or fetal morphological ultrasound has as main objectives : Confirm gestational age; Check the fetus for proper growth; Check the position of the placenta; Check the feasibility of multiple pregnancies; In some cases it is possible to know the sex of the baby . At this point, the fetus already has a dimension that allows a more accurate assessment of its anatomy, so this ultrasound can also detect some possible congenital malformations . The ultrasound only informs about the existence of possible physical anomalies and not about other (eg biochemical, metabolic, genetic, chromosomal, among others). Thus, a normal result in the ultrasound study does not guarantee that a child will be born without physical problems or impairments of body or psychomotor development. Also, although the 18-24 week ultrasound can detect fetal morphology abnormalities, the accuracy of this technique depends on: Gestation time - with higher reliability when ultrasound is performed between 21-23 weeks of gestation; Type of anomaly - since some have little or no ultrasound expression; Condition of the pregnant woman - obesity, very little amniotic fluid and application of fatty creams on the abdomen decrease the accuracy; Fetal position. Changes in fetal growth can be detected in the 18-24 week ultrasound. However, it may not be possible for the physician to: Identify the reasons why the changes occurred; Formulate a definitive diagnosis based only on the ultrasound results. Some diseases start or manifest in advanced stages of pregnancy, so its ultrasound diagnosis is necessarily late. This can happen, for example in: Infections; Some digestive abnormalities; Urinary obstructions; Intestinal obstructions; Skeletal dysplasias; Strokes. Third trimester ultrasound or fetal growth and well-being assessment The ultrasound to assess fetal growth and well-being is performed between 28 and 32 weeks of pregnancy. Its main objectives are: Detect fetal malformations that occur late during gestation; Assess normal growth and fetal well-being; Estimate fetal weight from different measurements. 3D-4D obstetric ultrasound This obstetric ultrasound allows you to observe the baby in real time . 3D and 4D obstetric ultrasounds allow the baby to be observed: In 3 dimensions (3D); In real time (4D). This ultrasound can be performed at any stage of pregnancy , although are more common after 26 weeks , due to the realism of the images that can be observed. Learn more about pregnancy surveillance . Meet the Obstetrics-Gynecology team from Hospital da Luz network. The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
