Table 4-1. Signs of Pregnancy

 

Presumptive	Unrelated to uterus or fetus	Amenorrhea
Probable	Related to uterus or mother’s	↑ uterine size
	feelings	b-hCG
Definitive	Related to the fetus	Sonogram of fetus
		Heard FHT

 

 

ESTABLISHING GESTATIONAL AGE

 

Conception Dating

 

Normal pregnancy duration postconception is 266 days or 38 weeks. However, most women can’t identify conception date accurately.

 

 

Menstrual Dating

 

Because the last menstrual period (LMP) is more easily identified than conception, preg-nancy duration in most cases is determined to be 280 days or 40 weeks from the LMP. We assume a 28-day menstrual cycle in which ovulation occurs on day 14 after the beginning of the LMP. Yet only 10% of women have a 28-day cycle. A normal cycle length can vary from 21 to 35 days.

 

 

Naegele’s Rule

 

Assuming 28-day cycles, a due date can be estimated as the LMP minus 3 months + 7 days.

 

 

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Table 4-2. Pregnancy Dating

Duration of pregnancy using:	Conceptional	266 days or 38 weeks
	dating
Duration of pregnancy using:	Menstrual dating	280 days or 40 weeks
Assumed cycle length		28 days
Calculate due date	Naegele’s rule	LMP–3 months + 7 days

Definition of abbreviations: LMP, last menstrual period.

 

OB Triad

Precise Day of Ovulation

 

• 21-day cycle: day 7

 

• 28-day cycle: day 14

 

• 35-day cycle: day 21

 

Basal Body Temperature (BBT)

 

The rise in BBT is assumed to be caused by the thermogenic effect of progesterone produced by the corpus luteum that formed after ovulation. The accuracy of BBT is ±1 week.

 

 

Menstrual History

 

Menstrual dating assumes ovulation occurred on day 14 after the first day of the LMP. However, normal menstrual cycles can vary from 21 to 35 days, making ovulation possible on day 7 to day 21. Because most women’s cycles are more or less than 28 days, adjustment of the due date may be necessary. Accuracy of menstrual dating is variable depending on the patient’s memory and record keeping. The accuracy of menstrual history is ±1 week.

 

		4 week cycles
	1	2	3	4
•	2 week proliferative phase
•	Ovulation on day 14
•	2 week luteal phase

Week cycles

 

 

1  2 3

 

• 1 week proliferative phase

 

• Ovulation on day 7

 

• 2 week luteal phase

 

Week cycles

 

1	2	3
				4	5

 

• 3 week proliferative phase

 

• Ovulation on day 21

 

• 2 week luteal phase

 

 

Figure I-4-1. Variations in Menstrual Cycle

 

 

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1

2

3

4

 

38 weeks from fertilization (true due date)

40 weeks from last menses (calculated due date)

 

With 4 week cycles, true due date equals calculated due date

 

 

1

2

3

 

38 weeks from fertilization (true due date)

 

40 weeks from last menses (calculated due date)

 

With 3 week cycles, true due date earlier than

 

							calculated due date
									38 weeks from fertilization
									(true due date)
	1		2	3		4	5
									40 weeks from last menses
									(calculated due date)
				With 5 week cycles, true due date later than
							calculated due date

 

Figure I-4-2. Effect of Cycle Length on Calculated Due Date

 

 

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