NU682 Unit 2 Pregnancy Dating Paper Example

Twelve Days Apart: A Unit 2 Pregnancy Dating Paper on Choosing Between a Remembered Period and a First Ultrasound

[Student Name]

Purdue University Global

NU682: Childbearing Family

Unit 2 Pregnancy Dating Paper

[Instructor Name]

[Date]

The patient and measurements are composite and illustrative; the sources cited are real.

The Two Dates

Ms. Ferreira stopped her combined pill in December to try to conceive. Her first period after stopping came in late January, and the next one began on March 1. She is certain of that date because it was the morning of a concert. Her cycles since stopping the pill have been 35 to 40 days long, rather than the 28 days the standard calculation assumes.

Counting 280 days from March 1 gives an estimated due date of December 6. Her first-trimester ultrasound, done because she had light spotting, finds one embryo, its heart beating, measuring 8 weeks 4 days from crown to rump. By her period she should be at 10 weeks 2 days. The difference is 12 days.

Which Measurement Is More Reliable

Dating by the last menstrual period assumes ovulation on about day 14 of a 28-day cycle. Many people ovulate earlier or later, and cycles after stopping hormonal contraception are often longer and irregular, as hers are. If she ovulated around day 26 of a 40-day cycle, conception would have occurred about twelve days later than the standard calculation assumes, which is exactly the difference the ultrasound found.

A crown-rump measurement taken in the first trimester is the most reliable way to set gestational age, usually within five to seven days (American College of Obstetricians and Gynecologists [ACOG], 2017). Early embryos grow at a very consistent rate, so their size reflects age closely. In a large cohort comparing dating methods, dates from the last period classified considerably more births as postterm than dates from early ultrasound, largely because of errors like the one in her case (Savitz et al., 2002).

Applying the Redating Thresholds

ACOG (2017) sets thresholds for when an ultrasound date replaces a period date. They depend on the gestational age at which the ultrasound is performed, because measurements become less precise as pregnancy advances. Before 9 weeks 0 days, a difference of more than 5 days means the ultrasound date should be used. From 9 weeks 0 days to 13 weeks 6 days, the threshold is more than 7 days. Thresholds widen in the second and third trimesters, reaching more than 21 days after 28 weeks.

Her ultrasound gestational age is 8 weeks 4 days, so the threshold is 5 days. The difference is 12 days, well beyond it. Her pregnancy is redated to the ultrasound: she is 8 weeks 4 days today, and her estimated due date moves from December 6 to December 18.

Once the first-trimester ultrasound sets the date, no later scan should move it.

The date is recorded in her chart as the best obstetric estimate, with the method noted. ACOG (2017) recommends that this estimate be determined as soon as data from the last period, first accurate ultrasound or both are obtained, and that it not be changed later except in rare circumstances. Later ultrasounds assess growth against this date. If a third-trimester scan measured small, it would raise the question of fetal growth restriction rather than a new due date.

What the Date Will Decide

The difference of twelve days seems small, but at least five later decisions depend on it.

Screening windows. Cell-free DNA screening can be done from 10 weeks. By her period she would have qualified today; by the ultrasound she should wait about ten days. Second-trimester serum screening, if chosen, has a defined window in which the results are interpreted against gestational age, and the wrong date distorts the risk estimate.

The anatomy scan. The detailed fetal anatomy ultrasound is scheduled around 18 to 22 weeks. With the period date it would be booked too early, when some structures are harder to see.

Glucose testing and vaccines. Screening for gestational diabetes at 24 to 28 weeks and Tdap vaccination between 27 and 36 weeks are timed by gestational age.

Term labels. Births are classified as early term from 37 weeks 0 days to 38 weeks 6 days, full term from 39 weeks 0 days to 40 weeks 6 days, late term from 41 weeks 0 days to 41 weeks 6 days and postterm from 42 weeks 0 days (Spong, 2013). These labels guide decisions about scheduled deliveries and induction.

Induction for a late pregnancy. Under the period date, she would reach 41 weeks on November 29 and might be offered induction for a late-term pregnancy when, by the more accurate date, she would be only 39 weeks 2 days. An induction that early for that reason would be unnecessary and could lead to a longer labor or a cesarean.

The Spotting

The ultrasound was done because of light spotting. The scan found the embryo inside the uterus with a heartbeat and no collection of blood behind the membranes. Light first-trimester bleeding is common and, with these findings, the outlook is reassuring. She is told to call for heavy bleeding, cramping like a period or passing tissue. Her blood type is checked; she is Rh positive, so no immune globulin is needed.

Explaining the Change to Her

Ms. Ferreira was upset at first: she had already told her family a December 6 due date and was sure of her period. The explanation focused on what the ultrasound measures rather than on whether she remembered correctly. Her period date was right; it was the assumption of a 28-day cycle that did not fit her. She found that reassuring. She was given a printed schedule built on the new date, showing when each test and vaccine will happen.

Documentation

The chart records the last menstrual period, the cycle length history, the crown-rump length and the ultrasound gestational age, the redating threshold applied and the final estimated due date with the method used. ACOG (2017) recommends documenting the basis of the due date, so that every clinician who sees her later uses the same date and knows why.

Conclusion

Ms. Ferreira's last period was accurate, but her long cycles meant it overstated her gestational age by twelve days. Her first-trimester ultrasound, measured before nine weeks, met the threshold for redating, and her due date moved to December 18. That single change protects every later decision in her pregnancy, from screening windows to the timing of any induction.

References

American College of Obstetricians and Gynecologists. (2017). Committee opinion no 700: Methods for estimating the due date. Obstetrics & Gynecology, 129(5), e150-e154. https://doi.org/10.1097/AOG.0000000000002046

Savitz, D. A., Terry, J. W., Dole, N., Thorp, J. M., Siega-Riz, A. M., & Herring, A. H. (2002). Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. American Journal of Obstetrics and Gynecology, 187(6), 1660-1666. https://doi.org/10.1067/mob.2002.127601

Spong, C. Y. (2013). Defining "term" pregnancy: Recommendations from the Defining "Term" Pregnancy Workgroup. JAMA, 309(23), 2445-2446. https://doi.org/10.1001/jama.2013.6235