Dating of pregnancy

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One reason for not dating pregnancies from the day of conception is that we cannot know that day exactly excluding cases of assisted reproduction , but we can know the first day of LMP, based on what a patient reports to us. Also, when providers all use the same LMP date, we are using one system that provides a standard convention. The average time between menstrual periods is about 28 days, and ovulation occurs about 14 days before the next period begins. The expected duration of a pregnancy is 40 weeks from the first day of LMP four weeks more than the traditional nine months by which many count.

It is not used by medical professionals to describe pregnancy length and is solely a political term. Sometimes we do use a trimester system if we are seeking to describe how far along a woman is in general terms.

How Doctors Date Pregnancies, Explained

Trimesters, however, have a variability of a few weeks, so they should be thought of as general benchmarks. The first trimester is generally considered to comprise the time up to 12 or 14 weeks from the LMP. The second trimester extends from the end of the first until about 26 to 28 weeks from the LMP, and the third trimester from the end of the second until delivery. Ultrasound can be used to date pregnancies, especially when the LMP is not known for example, pregnancy after a delivery but before a menses occurred or irregular menses without predictable ovulation.

Ultrasound dating is based on a series of measurements of the gestational sac in very early pregnancies and of the embryo or fetus itself as pregnancy progresses. However, unless there is a great difference between what is seen on an ultrasound and what was reported as LMP, doctors still use LMP to establish a due date. For example, if in the first trimester the ultrasound estimate falls within one week of the LMP estimate, we still use LMP to determine due date and length of pregnancy.

In the present study, first trimester measurements CRL and BPD 1 showed identical results regarding the duration of pregnancy median and SD and a mean difference of 0. In our regimen, we measure BPD routinely in the first trimester in order to detect central nervous defects. The dating methods are based on the assumption that all foetuses are of similar size at a given GA during the first half of the pregnancy, yet several factors, e. Like Saltvedt et al. The strength of the present study is that we studied a large unselected population in a standardised programme with a large number of first trimester measurements, thereby minimising potential selection bias.

Exclusion of post-term induction is a limitation to the study, causing an artificially low post-term rate. By using an older cohort this selection bias was minimized, since induction-rate in general is increasing. In our material the mean induction-rate was Defining the pregnancy length by mean, median or mode is widely debated. Mean values are affected by preterm births and also inductions due to post-term pregnancies; median values are not affected and therefore considered the most representative value estimating the general pregnancy length. We found a significant effect on change in post-term rate compared to different dating methods.

If we have had a lower induction rate in Denmark we are confident that the effect would be larger but it would not change our conclusion. In conclusion, we found an increase of the post-term rate in dating performed in first trimester compared to second trimester, caused by systematic discrepancies. When the formulas were calibrated, the post-term birth rates in the measurements performed in first trimester decreased.

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It is important to evaluate and correct the formulas used for dating to avoid misclassification of pre-term and post-term pregnancies and thus avoid unnecessary inductions due to post-term pregnancy. We thank Steen Rasmussen for his assistance with data management and Edwin Stanton Spencer for editing the language in the manuscript. Both were paid for the service through project funds. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. National Center for Biotechnology Information , U. Published online Jan Author information Article notes Copyright and License information Disclaimer.

The authors have declared that no competing interests exist.

Also of Interest

Received Jun 18; Accepted Dec This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Number of inductions, caesarean section and spontaneous deliveries per year. Abstract Objectives To evaluate in a national standardised setting whether the performance of ultrasound dating during the first rather than the second trimester of pregnancy had consequences regarding the definition of pre- and post-term birth rates. Results The change from use of second to first trimester measurements for dating was associated with a significant increase in the rate of post-term deliveries from 2.

Conclusions Systematic discrepancies were identified when biometric formulas were used to determine duration of pregnancy. Introduction The accurate dating of the duration of pregnancy is of importance in regard to prenatal care since several clinical decisions are based on gestational age GA. Open in a separate window.

Results There were up to three days differences in median gestational age depending on the method employed. Table 1 Mean, median, and mode of Pregnancy length days in relation to dating method. Table 2 Measurement errors when methods were compared to first and second trimester measurements. Table 3 Number of pre-term, term, and post-term pregnancies in relation to dating method.

The distribution of gestational age at birth in relation to dating method. Table 4 Number of pre-term, term, and post-term pregnancies in relation to dating method after calibration of formulas. Each method is compared with CRL. Discussion In a large cohort of pregnant women, the use of first trimester dating resulted in significantly more post-term pregnancies compared to second trimester dating.

Supporting Information S1 Table Number of inductions, caesarean section and spontaneous deliveries per year. DOCX Click here for additional data file. Acknowledgments We thank Steen Rasmussen for his assistance with data management and Edwin Stanton Spencer for editing the language in the manuscript. Taipale P, Hiilesmaa V. Predicting delivery date by ultrasound and last menstrual period in early gestation. The performance of a first-trimester ultrasound is not always possible.

Patients will occasionally initiate prenatal care in the second trimester, or they may not present to a facility with ultrasound capability. Ultrasound between 18 to 20 weeks will allow both optimal dating criteria and detailed anatomical survey of the fetus. Head measurements should be performed using a transverse section of the head at the level of the thalamus and cavum septum pellucidum.

The cerebellar hemispheres should not be visible during measurement of biparietal diameter or head circumference. Head measurements involve the bone, not skin line.

The abdominal circumference should be measured in transverse section at a plane in which vertebra, stomach, umbilical vein, and portal sinus are visible. For abdominal circumference, skin line should be used for measurement. The femur length measurement should include the full length of the femur perpendicular to the ultrasound probe. The distal femoral epiphysis should not be included in the femur length measurement.

As an individual measurement, either the head circumference or biparietal diameter is the best predictor of gestational age. The earlier in the second trimester that an ultrasound is performed, the more accurate gestational age measurement. One major concern with third trimester dating ultrasound is underestimating the gestational age of a growth-restricted fetus.

Management decisions based on third-trimester ultrasound alone can be difficult for this reason. With improving technology and research, assisted reproductive technology is more widely used to establish pregnancy in patients with infertility. If a patient has become pregnant as a result of in-vitro fertilization, the age of the embryo and the date of transfer should be used to date the pregnancy.

Associated Data

As stated above, if a pregnancy does not have an ultrasound to confirm or establish an estimated delivery date before 22 weeks 0 days, that pregnancy should be considered suboptimally dated. The major complication with suboptimally dated pregnancies is the lack of accuracy of third-trimester ultrasound.

If a fetus is macrosomic, for example in a patient with uncontrolled diabetes, third-trimester ultrasound can overestimate gestational age.

How is a dating scan performed?

For this reason, the timing of delivery in a suboptimally dated pregnancy can be a difficult decision for the clinician. Establishing an accurate gestational age and estimated delivery date is the most important step in the management of any pregnancy. Accurate knowledge of gestational age allows laboratory and screening tests to be performed at the appropriate time in the pregnancy. Suboptimally dated pregnancies, due to the error of ultrasound at advanced gestational age, can be difficult to manage because of the uncertainty of the pregnancy dating.

There is no role for elective delivery in suboptimally dated pregnancies.

If the fetus is earlier in gestation than estimated by third-trimester ultrasound, elective delivery could impose increased risk of neonatal morbidity and mortality. In pregnancies with indications for delivery pre-eclampsia, gestational diabetes, etc. Amniocentesis for fetal lung maturity should not be routinely used before planning delivery for suboptimally dated pregnancy. Even with proven fetal lung maturity, late preterm and early term infants have an increased risk of respiratory morbidity. Elective delivery is indicated at 41 completed weeks, due to concerns that the fetus could be further along than estimated by third-trimester ultrasound.

For this reason, consideration can be given to initiating antepartum fetal surveillance between 39 and 40 weeks. In patients with prior cesarean delivery who desire repeat cesarean section, repeat cesarean should be performed at 39 weeks using the best assessment of gestational age.

To access free multiple choice questions on this topic, click here. This book is distributed under the terms of the Creative Commons Attribution 4. Turn recording back on. National Center for Biotechnology Information , U. StatPearls Publishing; Jan-. Show details Treasure Island FL: StatPearls Publishing ; Jan-. Pregnancy, Dating John A. Author Information Authors John A. Affiliations 1 Louisiana State University.

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