CRL dating is the best method from 8 6/7 to 13 6/7 weeks. One should not use MSD if you can calculate the crown-rump length (CRL).

Additionally, anembryonic pregnancies can have a measurable MSD. MSD should not be used for final due date estimation. MSD increases by 1mm per day during early pregnancy. Calculation of EDD is from MSD by adding 30mm. To calculate the MSD, the diameter sac measurement must be in the perpendicular plane, with a mean of three measurements. Mean sac diameter (MSD) can be used early in gestation to estimate the due date if there is no embryo on the sonogram.

An embryo is usually seen later between 5 to 6 weeks. A Transvaginal (TV) sonogram can identify an intrauterine pregnancy (IUP) approximately 4 weeks after the LMP. Ultrasonography is best within the first half of the pregnancy. If a patient cannot recall the first day of her last menstrual period, has irregular cycles, or Naegele’s rule cannot be used, the next step is ultrasonography. It is important to understand all the clinical estimators of calculating the due date have a margin of error of +/- 3 weeks. Electronic Doppler device can detect fetal heart tones as early as 8 to 10 weeks gestation. Other factors that decrease accuracy include body habitus, multiple gestations, and a retroverted uterus.įetal heart tones are auscultated with the fetoscope at 19-20 weeks.

For patients who have had a previous cesarean, the uterus is generally in a higher position because of pelvic adhesions. For instance, if a patient has uterine fibroids, this can decrease the accuracy of this technique. Even though this method is informative, it is subject to error secondary to anatomical and/or structural changes of the uterus. The over-the-curve technique is most practical from 16 to 38 weeks with a +/- 3 cm margin of error. Clinical landmarks approximate a 20-week size gestation at the umbilicus, with a 16-week size gestation at the halfway mark between the symphysis and the umbilicus. Highly trained practitioners can palpate the uterine fundus as early as 12 weeks, commonly at the symphysis. The over-the-curve technique involves using a measuring tape in centimeters from the superior aspect of the pubic symphysis to over the top of the uterine fundus. Other clinical methods that can calculate EDD include uterine size by pelvic examination using the over-the-curve technique. Parikh’s formula calculates EDD by adding nine months to the start of the last menstrual period, subtracting 21 days, then add the duration of the previous cycle. Last, this method cannot differentiate between menstrual bleeding and early pregnancy bleeding. Parikh’s formula was created to account for differences in menstrual cycle length. Third, there are small variations in the duration between fertilization and blastocyst implantation. Second, this method assumes a woman’s cycle is exactly 28 days, with ovulation occurring at day 14, however, it does not consider menstrual cycles with shorter or longer durations. First, a woman may not accurately recall the first day of her menstrual cycle. There are several fallacies with Naegele’s rule. These factors are used to determine the length of her cycles and ovulation period. It is prudent for the obstetrician to get a detailed menstrual history, including duration, flow, previous menstrual periods, and hormonal contraceptives. Naegele’s rule, derived from a German obstetrician, subtracts 3 months and adds 7 days to calculate the estimated due date (EDD). Historically, an accurate LMP is the best estimator to determine the due date. An average pregnancy lasts 280 days from the first day of the last menstrual period (LMP) or 266 days after conception.

Conception date to due date