Early obstetric ultrasound (5-10 weeks of gestation): values, measurements and heartbeat frequency

Once the pregnancy has been discovered and the pregnancy tests have been done, the next step is the first ultrasound. If it is performed between the 5th and 10th week of pregnancy, it is also called early ultrasound and is used to view the implantation in the uterus, the size (and any number) of the gestational chambers.

Depending on the week in which it is performed, it can also highlight the presence and number of embryos, cardiac activity and carry out a reliable dating of the pregnancy .

The first sonographic sign of a pregnancy that has just begun is the visualization of a gestational chamber in the uterus.

The gestational chamber can be visible as early as 4-5 weeks after the last menstruation.

In the 5th week, the chorionic cavity reaches a diameter of 5-6 mm and corresponds to a Beta HCG value of approximately 800-1000 IU/l .

The egg chamber grows at a successive rate of about 1 millimeter per day.

13 days after conception, the primary yolk sac transforms into a secondary sac which performs an important nutritional function for the embryo. The secondary yolk sac can be visualized via transvaginal ultrasound starting from the 5th week of pregnancy. This structure appears rounded inside the gestational chamber.

The visualization of the secondary yolk sac in this gestational period is very important as it allows to exclude the clear egg syndrome .

From the 6th week it is possible to visualize the embryo surrounded by the amniotic membrane and located at the opposite pole with respect to the secondary yolk sac.

Embryonic cardiac activity can already be highlighted at the 6th week of pregnancy thanks to the development of the cardiovascular system.

The fetal heart rate should always be visualized when the cephalo-breech length is 5mm .

Another structure that can be seen in early pregnancy is the amniotic cavity  which is initially visible as a rounded structure within the gestational chamber. The first ultrasound demonstration of the amniotic membrane close to the embryonic pole occurs starting from the 6th week of gestation, when the diameter of the amniotic cavity is equal to 1 cm. This value increases rapidly and we find it tripled a week later.

What value should the fetal heartbeat have?

Heart rate varies according to gestational age.

  • At 6 weeks it is 90-110 bpm (beats per minute)
  • around the 9th week it increases up to 140-170 bpm
  • at the 14th week the heartbeat is around 150 bpm

How is gestational age determined?

The most reliable method for estimating the weeks of pregnancy in the first trimester is based on the cephalo-breech length of the embryo/foetus (CRL= Crpwn-Rumple Length). This value in 95% of cases has an accuracy of ± 3-4 days.

This measurement must be done before the 11th week of gestation. Subsequently, its detection loses reliability for the purpose of dating the pregnancy due to the physiological bending attitude that the fetus tends to assume. For this reason, after the 11th week, the most reliable parameter for assessing gestational age is represented by the measurement of the biparietal diameter (DBP).

Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.

Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.

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