Pregnancy Dating Guidance

Abstract

Accurate estimation of pregnancy duration is crucially important for the definition of term and preterm birth and informs care provision throughout pregnancy. However, this remains a challenge in many resource-limited context largely because the reliability of recall of the last menstrual period is low, and access to early ultrasound to measure foetal crown-rump length and other foetal biometry is highly limited. This chapter outlines a practical simplified approach to estimating pregnancy duration early following engagement with ANC. This forms the basis of subsequent care provision to correctly identify preterm labour/birth, improve epidemiological data on preterm low birth weight incidence, and enable proper triaging of care and its provision.

1 Background

An essential component of antenatal care is pregnancy dating, allowing for an accurate estimation of the duration of pregnancy. It is important for identifying the optimum timing of obstetric interventions such as location of birth, delivery mode, and management of foetal growth abnormalities. Accurate pregnancy dating improves the classification of preterm birth (PTB) and enables global PTB rates to be comparable. In low- and middle-income countries (LMICs), where the burden of PTB and intrauterine growth restriction is highest [1, 2], pregnancy dating is a challenge: women are often unable to recollect their last menstrual period, and menstrual cycle lengths vary due to short birth intervals and lactation [3, 4]. Clinical palpation to estimate uterine size is often inaccurate and influenced by foetal growth restriction, uterine fibroids, foetal malpresentation (associated with high parity), and maternal obesity. Late presentation for pregnancy registration is common in LMICs making pregnancy dating a challenge.

This guidance details the optimum approach to pregnancy dating utilising the best resources currently available in different contexts and taking into account late presentation.

2 Evidence Statement

Accurate pregnancy dating is important to enable accurate diagnosis of preterm labour and delivery. It varies with the duration of the pregnancy at presentation.

In early pregnancy, a reliable last menstrual period (LMP) should be employed and confirmed by the foetal crown-rump length (CRL) if ultrasound is readily accessible. A discrepancy of more than 1 week between both modalities should trigger a switch in the confirmed pregnancy duration and expected date of delivery to the ultrasound CRL as this is more reliable.

After the first trimester, foetal biometry using a formula (algorithm that assesses BPD/HC/FL) may be employed if ultrasound is readily available. If ultrasound is not immediately accessible, clinical assessment of the uterine fundal height should be used pending confirmation by ultrasound where possible. Foetal biometric estimation of gestational age at 20–24 weeks’ is further improved if the transcerebellar distance can be employed either singly or with femur length assessment to estimate the duration. Where ultrasound is unavailable, then the symphysiofundal height should be used.

3 Synopsis of Best-Evidenced Pregnancy Dating Methods

3.1 Last Menstrual Period

This is the most widely used method to estimate pregnancy duration. If known with certainty, it offers a good estimation of the baby’s due date and accurate pregnancy dating. However, it may overestimate pregnancy duration by more than 3 days in high-income settings (HICS) [5] and longer in LMICs [6]. It is dependent on the regularity of the menstrual cycle and subjective recall of the first day of the last period.

(Moderate to high certainty of evidence)

3.2 First Trimester Ultrasound

Measurement of the foetal crown-rump length (CRL) is considered to be the gold-standard method for estimating gestational age (up to 14 weeks’ gestation) [5, 7]. Unfortunately, in LMICs, ultrasound early in gestation is often not universally available, and there is the tendency for pregnant women to present late for antenatal care. These issues limit the application of CRL measurement in these settings.

(Moderate to high certainty of evidence)

3.3 Ultrasound Standard Foetal Biometric Measurements at 14 to 20 Weeks’ Gestation

Standard biometric measurements (biparietal diameter, head circumference, abdominal circumference, and femur length) provide an accurate estimation of gestational age (to within ±1–2 weeks of the crown-rump length (CRL) measurement of gestational age) [8].

(Moderate to high certainty of evidence)

3.4 Ultrasound Standard Foetal Biometric Measurements after 20 Weeks’ of Gestation

Standard biometry does not perform as well as it does at less than 20 weeks’ gestation, with accuracy of only ± ≥ 3 weeks of the CRL measurement [7], especially in LMICs where 19·3% of infants are born small for gestational age [2]. Measurement of the cerebellum alone or combined with femur length [9] provides more accurate estimation of gestational age compared with standard biometry measurements [10].

(Moderate to high certainty of evidence)

3.5 Symphysiofundal Height Estimation

In late pregnancy after 20 weeks, this provides gestational age estimation comparable with the last menstrual period and may be employed against a validated normogram when women present late and menstrual dates are not reliable and access to ultrasound is limited [11].

(Low certainty of evidence)

Figure 1 outlines the pragmatic steps that facilitate estimating pregnancy duration as accurately as possible in low as well as high resource settings, based on careful evaluation of the last menstrual period history, the availability and utilisation of ultrasound, as well as the best ultrasound parameters that should be employed. In limited resource settings where late booking is rife, it highlights the use of clinical estimation of the symphysiofundal height to augment available information about pregnancy duration.

Fig. 1

Flow diagram to guide pregnancy dating. EDD Estimated date of delivery; CRL Crown-rump length, HC Head circumference, BPD Biparietal diameter; FL Femur length