The estimated due date (EDD or EDC) is the date that spontaneous onset of labor is expected to occur. The due date may be estimated by adding days (9 months and 7 days) to the first day of the last menstrual period (LMP). Ultrasound uses the size of the fetus to determine the.
Table of contents
- Methods for Estimating the Due Date - ACOG
- Women's Health Care Physicians
- How is a dating scan performed?
Doctors test a sample of urine or sometimes blood from the woman to determine whether she is pregnant. One of these tests, called an enzyme-linked immunosorbent assay ELISA , can quickly and easily detect even a low level of human chorionic gonadotropin in urine. Some tests can detect the very low level that is present several days after fertilization before a menstrual period is missed.
Results may be available in about half an hour. During the first 60 days of a normal pregnancy with one fetus, the level of human chorionic gonadotropin in the blood approximately doubles about every 2 days. These levels can be measured during the pregnancy to determine whether the pregnancy is progressing normally.
Pregnancies are conventionally dated in weeks, starting from the first day of the last menstrual period. The doctor calculates the approximate date of delivery by counting back 3 calendar months from the first day of the last menstrual period and adding 1 year and 7 days. For example, if the last menstrual period was January 1, the doctor counts back 3 months to October 1, then adds 1 year and 7 days.
The calculated due date is October 8 the next year. Delivery between 3 weeks before and 2 weeks after the calculated date is considered normal. Consequently, the embryo is about 2 weeks younger than the number of weeks traditionally assigned to the pregnancy. In other words, a woman who is 4 weeks pregnant is carrying a 2-week-old embryo. Pregnancy lasts an average of days 38 weeks from the date of fertilization conception or days 40 weeks from the first day of the last menstrual period if the woman has regular day periods.
Pregnancy is divided into three 3-month periods, based on the date of the last menstrual period:. The most accurate way to determine when a baby is due is ultrasonography, particularly if it is done during the first 12 weeks. Tap to switch to the Professional version. Detecting and Dating a Pregnancy. Additional Content Medical News. This is the Consumer Version. Click here for the Professional Version. The most important step in the initial evaluation of any pregnant patient is establishing an accurate delivery date due date. Accurate knowledge of the gestational age is important for numerous reasons.
For example, certain antenatal screening tests like the quadruple marker screen screening test for fetal aneuploidy and open neural tube defects must be performed with accurate knowledge of the gestational age for an accurate assessment of lab values. In patients with a history of preterm labor and delivery, screening tests and interventions early in pregnancy can be used to prevent preterm labor in a subsequent pregnancy.
As pregnancy progresses, accurate and optimal pregnancy dating is important when deciding on the timing of both indicated and elective deliveries. Accurate gestational age determination has been found to affect pregnancy outcomes. A Cochrane review concluded that first-trimester ultrasound dating could reduce the need for post-term induction of labor and lead to earlier detection of multiple gestations. Pregnancy ultrasound involves an anatomic survey of uterus and adnexa. If one sees a fetal pole, record the presence of fetal heart tones in the medical record.
Methods for Estimating the Due Date - ACOG
In first trimester ultrasound, it may be possible to visualize the ovarian size and presence or absence of ovarian cysts. Beyond 18 weeks of gestation, a full fetal anatomic survey is possible. For appropriate management of any pregnancy, practitioners must establish gestational age. Any pregnancy that does not meet this criterion should be considered suboptimally dated. Ultrasound has been used in obstetrics for over 50 years and is considered safe. Ultrasound does have effects on tissues due to energy used to obtain images.
For this reason, only use ultrasound when it is clinically indicated and for the shortest amount of time possible. As previously described, there are multiple methods for establishing an estimated delivery date. When using the patient's last menstrual period to establish pregnancy dating, Naegele rule requires the use of a calendar. A physical exam is an alternate method for establishing gestational age, which requires the use of an appropriately suited exam table with stirrups for a reliable pelvic exam.
For fundal height measurement, a non-elastic centimeter tape measure is needed to measure the pubic symphysis to fundus measurement. Ultrasound is the most reliable method to establish pregnancy dating, particularly first-trimester ultrasound. Transvaginal ultrasound requires the use of a probe cover, and latex or nonlatex may be used. Beyond eight weeks, transabdominal ultrasound is typically satisfactory for evaluation of pregnancy. Abdominal obesity or a retroverted uterus may cause difficulty during transabdominal approach.
A gel warmer may be used for patient comfort, but caution should be used if the warm gel is used transvaginally to ensure the gel is not too hot. In certain circumstances, an ultrasound technician will be the first person to evaluate a pregnancy using ultrasound. Patient preparation before ultrasound varies depending on which approach is used. For transabdominal ultrasound, a full bladder is helpful but not required. Full bladder acts as an acoustic window and improves image details.
For a transvaginal ultrasound, a full bladder can displace the uterus posteriorly and out of the field of view of the transvaginal ultrasound probe.
Women's Health Care Physicians
For this reason, it is recommended to perform transvaginal ultrasound with an empty bladder. A recommended approach would be to perform transabdominal ultrasound with a full bladder, if the transvaginal approach is needed, have the patient empty her bladder before a transvaginal ultrasound. Ultrasound should be performed on a bed or stretcher which is equipped for performing pelvic exams with foot stirrups and adjustable lower half of table. Abdominal ultrasound approach may be performed in the supine position. The transvaginal approach is best carried out with the patient in the lithotomy position, with the patient's buttocks at the end of the table allowing for a complete range of motion with the transvaginal ultrasound probe.
One method of estimating the delivery date in this instance is by using Naegele Rule. This technique assumes that the patient has a normal day menstrual cycle and ovulates on day 14 of that cycle. If the patient is unsure of her last menstrual period or has irregular menstrual cycles, the accuracy of this method decreases.
In many instances, a patient will not remember the first day of her last menstrual cycle or will be unsure of the exact date on which her cycle started. In this circumstance, a thorough physical examination will allow a preliminary estimation of the gestational age until ultrasound can be performed. Obviously, there are wide variations in uterine size at any given age due to gynecologic pathology e.
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- Methods for Estimating the Due Date?
Fruit sizes are extremely variable, so this model is widely inaccurate in certain circumstances. The five-week pregnant uterus is approximately the size of a small, unripe pear. The six-week pregnant uterus feels like a small orange. The eight-week pregnant uterus feels like a large orange. The week pregnant uterus feels like a grapefruit. Many obstetricians develop a feel for uterine size through the clinical experience of performing thousands of bimanual exams and have their comparisons for estimating gestational age in this manner.
This method can be performed before ultrasound examination to determine the urgency for which dating ultrasound should be performed and ensure the patient has optimal pregnancy dating. Fundal height measurement is another physical exam parameter that can be used to estimate gestational age. The distance from the uterine fundus to the pubic symphysis defines fundal height measurement.
Measurement should be performed using a non-elastic tape measure, and the patient should have an empty bladder. The most common use for fundal height measurement is to record the trend to assess appropriate fetal growth and to screen for fetal growth restriction. The usefulness of fundal height measurement in any circumstance has varied widely across the literature but can be helpful in resource-poor areas for an estimation of gestational age.
The assumption that is made with fundal height measurement is that each 1 cm increment in fundal height corresponds to one week of gestation. Fundal height measurement is most useful after 20 weeks when the uterine fundus is usually at the level of the umbilicus. A difference of two to three weeks or two to three centimeters in either direction is within the normal range. For example, a patient whose fundal height is 25 cm can have a gestational age range from 23 to 27 weeks.
How is a dating scan performed?
The assumption for this patient will be that she is approximately 25 weeks of gestation. This measurement can vary widely based on different maternal variables. Uterine fibroids, amniotic fluid abnormalities, increased maternal body mass index BMI , and fetal growth abnormalities are some examples of circumstances that can alter the accuracy of fundal height measurement. First-trimester ultrasound can be performed either transvaginally or transabdominally.