First Trimester Ultrasound
Obstetric Ultrasound for Viability, Age, and Maternal Health
Aug 22, 2009
Early pregnancy ultrasounds are becoming more common, although they are not done for all pregnancies. Some clinics do not do first trimester ultrasound unless a miscarriage or other problem is suspected. Some regularly use it to confirm the due date for a pregnancy. It may be done for the mother's peace of mind, if her health is a concern, or if the age of the pregnancy is unknown.
Viability
Vaginal spotting is quite common in early pregnancies and may be a cause for concern regarding miscarriage. If a possible miscarriage is suspected, a first trimester ultrasound is done to confirm the viability of the baby. Doppler ultrasound is used to acquire a heart beat.
Miscarriage may be accompanied by a lot of pain and bleeding. Some miscarriages are without signs and symptoms. Ultrasound can be used to determine if a miscarriage is complete, meaning the contents of the pregnancy are expelled from the uterus, or incomplete, meaning the embryo is not viable, yet the gestational sac remains in its proper place within the uterus.
Dating
The age of a pregnancy is often unknown. A first trimester ultrasound is done to determine the age of the embryo and the due date. This is done by acquiring a crown-rump length, which is the most accurate method of dating a pregnancy. The date determined will be used for comparison when the second trimester ultrasound is done.
Pain and Bleeding During Pregnancy
Most women experience some discomfort or pain with early pregnancy. The pregnancy is accompanied by an ovarian cyst called a corpus luteum. The corpus luteum is the ruptured follicle that released the egg during ovulation. This cyst releases progesterone throughout the first trimester to sustain the pregnancy until the placenta matures at week 12 and is often a source of pain. It is identified using transvaginal ultrasound during the first trimester.
Subchorionic hemorrhage often occurs in the first trimester and is a common cause of bleeding during pregnancy. It is usually insignificant and is of no consequence to the pregnancy. Subchorionic hemorrhage may be the result of bleeding resulting from the implantation of the egg to the uterine wall. It is commonly seen during first trimester ultrasound.
Ectopic Pregnancy
An ectopic pregnancy occurs when the pregnancy is not within the uterus. It is most likely found within the fallopian tube, on the ovary, or by the right kidney. An ectopic pregnancy is confirmed when a gestational sac is seen outside of the uterus. This is not always the case, however, as bowel can obstruct the view of the gestational sac. Generous amounts of fluid within the pelvis may be a sign of ectopic pregnancy. A pseudo-sac may be seen within the uterus. A pseudo-sac is an empty sac within the uterus. Visualizing an early pregnancy is difficult due to its small size, and ultrasound cannot be used to see through bowel, so several ultrasounds may be done to confirm an ectopic pregnancy.
The Ultrasound Scan
First trimester ultrasound follows the same protocol as pelvic ultrasound in addition to some measurements for the pregnancy. A full bladder will be required for the abdominal portion of the exam, and specific instructions must be followed. Both abdominal and transvaginal ultrasound may be used. The uterus and ovaries are examined for any problems that may occur during pregnancy. The corpus luteum cyst is usually visualized on one of the ovaries.
The gestational sac is measured. Most ultrasound machines give a due date based on the gestational sac size, although this measurement is often quite inaccurate. The yolk sac feeds the embryo until the placenta matures. It is measured and is expected to be less than four millimeters in diameter. The embryo is measured from the top of the head to the rump. This is called a crown-rump length and is the most accurate measurement for determining embryonic age.
Early pregnancy ultrasound can be done to assess the health of the mother or the baby. It cannot be done until the second half of the first trimester because the embryo is too small to be seen with ultrasound until about week six or seven of gestation.
Resource
Ultrasound in Obstetrics And Gynecology: Textbook and Atlas, Volume 2
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