Until recently, clinicians customarily calculated menstrual age with term pregnancy averaging approximately 280 days, or 40 weeks between the first day of the LMP and birth. This corresponds to 9 and 113 calendar months. However, menstrual cycle length variability among women renders many of these calculations inaccurate. This realization, combined with the frequent use of first-trimester sonography, has led to more accurate gestational age determination (Duryea, 2015). Much of this change hinges on the accuracy of early sonographic measurement. As a result, the American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal-Fetal Medicine (Reddy, 2014) together recommend the following: 1.
First-trimester sonography is the most accurate method to establish or reairm gestational age.
2.
In conceptions achieved with assisted-reproductive technology, this gestational age is used.
FIGURE 7-1 Terminology used to describe pregnancy duration.
3.
If available, the gestational ages calculated from the LMP and from first-trimester sonography are compared, and the estimated date of coninement (EDC) recorded and discussed with the patient.
4.
The best obstetrical estimate of gestational age at delivery is recorded on the birth certiicate.
he embryofetal crown-rump length in the first trimester is accurate ±5 to 7 days. hus, if sonographic assessment of gestational age difers by more than 5 days prior to 9 weeks' gestation, or by more than 7 days later in the irst trimester, the estimated delivery date is changed.
An EDC based on LMP can be quickly estimated as follows: add 7 days to the irst day of the LMP and subtract 3 months. For example, if the irst day of the LMP was October 5, the due date is 10-05 minus 3 (months) plus 7 (days) = 7-12, or July 12 of the following year. This calculation has been termed the Naegele rule. The period of gestation can also be divided into three units of approximately 14 weeks each. These three trimesters are important obstetrical milestones.
In addition to estimating the EDC with either Naegele rule or pregnancy "wheels," calculator tools in the electronic medical record and smartphone applications can provide a calculated EDC and gestational age. For example, the American College ofObstetricians and Gynecologists (2016) has developed a calculator application that incorporates sonographic criteria and the LMP or embryo transfer date. This is discussed further in Chapter 10 (p. 183).
The complexity of embryofetal development is almost beyond comprehension. Figure 7-2 shows a developmental sequence of various organ systems. New information regarding organ development continues to accrue. For example, imaging techniques help unravel the contributions of gene regulation and tissue interaction to eventual three-dimensional organ morphology (Anderson, 2016; Mohun, 2011). Others have described the sequence of gene activation that underlies cardiac development.
During the irst 2 weeks after ovulation and then fertilization, the zygote-or preembryo-develops to the blastocyst stage. The blastocyst implants 6 or 7 days following fertilization. The 58-cell blastocyst diferentiates into ive embryo-producing cells-the inner cel mass-and the remaining 53 cells form placental trophoblast. Details of implantation and early development of the blastocyst and placenta are described in Chapter 5 (p. 87).
Ears Canals,
Diaphragm Transverse septum, diaphragm
Lungs Tracheoesophageal septum, bronchi, lobes
Heart Primitive tube, great vessels, valves. chambers
Intestines Foregut, liver, pancreas, midgut gut chlea. inner ears. ossicles --Yolksac . Developing