Ob sonogram for dating
The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that pregnant women have routine obstetric ultrasounds between 18 weeks' and 22 weeks' gestational age (the anatomy scan) in order to confirm pregnancy timing, to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy, and to assess for congenital malformations and multiple pregnancies (twins, etc).
Additionally, the ISUOG recommends that pregnant women have obstetric ultrasounds between 11 weeks' and 13 weeks 6 days' gestational age in countries with resources to perform them (the nucal scan).
Obstetric ultrasonography is the use of medical ultrasonography in pregnancy, in which sound waves are used to create real-time visual images of the developing embryo or fetus in its mother's uterus (womb).
The procedure is a standard part of prenatal care in many countries, as it can provide a variety of information about the health of the mother, the timing and progress of the pregnancy, and the health and development of the embryo or fetus.
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However, in the first trimester there is very little variation in fetal size, and so it turns out that an ultrasound done between 7 and 13 weeks is the most accurate. Compare the maximum error that each ultrasound can potentially have, and you'll see what I mean: This means that a pregnancy for which no period dates are available, and who did not get an exam or ultrasound until the third trimester, can have an EDC which could conceivably fall within a range as broad as 42 days! The main situation where this can cause a bit of friction between provider and patient is when the patient has uncertain dates, and/or a late ultrasound, and feels like she's close enough to full term to ask for her labor to be induced, yet she has a cervix which is unripe, or unfavorable for induction.
This can sometimes lead, if one isn't careful, to unnecessary induction of labor, which can result in unnecessary cesarean section, or delivery of a premature baby, who then requires transfer to a special care nursery.
In the past, the EDC was calculated by using Naegele's Rule, which determined the date by subtracting 3 months from the 1st day of the last period and then adding 7 days.
The bright white circle center-right is the head, which faces to the left.
Features include the forehead at 10 o'clock, the left ear toward the center at 7 o'clock and the right hand covering the eyes at .
However, it is extremely common to encounter patients who have irregular or infrequent menstrual cycles, or have fibroid tumors that cause their uterus to feel enlarged on pelvic exam, or who conceive shortly after a pregnancy ends (without ever actually having had a period after the last pregnancy), or who got pregnant while taking birth control pills, and these situations often render the above methods useless and misleading when trying to figure out a reliable EDC.
This is where our wonderful ultrasound machines can make a crucial difference.
There is no difference, however, in perinatal death or poor outcomes for babies.