gestational age

ESTIMATION OF

GESTATIONAL AGE

By: Anushya Jayendran

OBJECTIVES

• Definitions of important terminology

• Methods of determination of gestational age

• Importance of determination of gestational age

TERMINOLOGY

• Gestational Age– Refers to the length of pregnancy after the first

day of the last menstrual period (LMP) and is usually expressed in weeks and days.

• Conceptional Age– Refers to the true fetal age and refers to the

length of pregnancy from the time of conception.

DETERMINATION OF GESTATIONAL AGE


History Taking

Physical Examination Investigation


HISTORY TAKING

Date of Fruitful Coitus

Date of Last Normal Menstrual

Period(Naegele’s Formula)

Date of Quickening

HISTORY TAKING

• Date of Fruitful Coitus– 266 days are to be added to the date of single

fruitful coitus – Accuracy is up to 50% within 7 days on either side– Advantages: Useful and extremely accurate when

used in cases of assisted reproduction, particularly in IVF

– Disadvantages: Not practicable except when pregnancy occurs in instances of sudden death or absence of the husband or rape

HISTORY TAKING• Date of Last Normal Menstrual Period

– This follows Naegele’s Rule: EDD is 280 days from the LNMP in a patient with a regular cycle of 28 days and where ovulation occurs in midcycle

– Accuracy is up to 50% within 7 days on either side– If the interval of cycles is longer, the extra days are to be added

and if the interval is shorter, the lesser days are to be subtracted to get the EDD

– Advantages: Easy to calculate in a patient who is certain of LNMP– Disadvantages: Inaccurate in patients in the following

circumstances:-• Oligomenorrhoea or polymenorrhagia (irregular cycles)• Bleeding in the first trimester of pregnancy (implantation bleeding)• Pregnancy following the use of oral contraceptives or intrauterine devices• Pregnancy in the postpartum period (lactational amenorrhoea)

HISTORY TAKING

• Date of Quickening– A rough idea about the probable date of delivery

can be deduced by adding 22 weeks in primigravidae and 24 weeks in multiparae to the date of quickening

– Advantage: Can be used as an estimation in the absence of other more reliable methods

– Disadvantages: Inaccurate as every patient is different and values used are based on an average


PHYSICAL EXAMINATION

Uterus

Size of Uterus

Height of the Uterus

Cervix From Vaginal Examination

Fetus

Palpation of Fetal Parts

Auscultation of Fetal Heart Rate

Lightening

Size of Fetus


• Uterus– Size of Uterus

• Can be assessed by vaginal examination• Is most accurate prior to 12 weeks• Corresponds with the Period of Amenorrhoea• Disadvantages: Size can be misleading in the

presence of multiple pregnancy, uterine fibroids, or a full bladder

– Height of the Uterus• Refers to the Symphysial-Fundal-Height (SFH)• Corresponds with the Period of Gestation

(POG)• Advantages: Accurate in cases of normal

pregnancy• Disadvantages: Inaccurate in cases of fetus

which are small for gestational age or pregnancies with inadequate liquor


• Cervix– From Vaginal Examination• If the cervix becomes shorter and dilated, the labour is

fairly not far off• More accurately assessed using the Bishop’s score• Advantages: Extremely accurate when used to monitor

the progress of labour• Disadvantages: Can only be used to assess pregnancy in

the later part. Labour may start even with long and closed cervix

BISHOP SCORE

• Scoring– Cervical Dilation

• Cervix dilated < 1 cm: 0• Cervix dilated 1-2 cm: 1• Cervix dilated 2-4 cm: 2• Cervix dilated > 4 cm: 3

– Cervical Length (Effacement)• Cervical Length > 4 cm (0% effaced): 0• Cervical Length 2-4 cm (0 to 50% effaced): 1• Cervical Length 1-2 cm (50 to 75% effaced): 2• Cervical Length < 1 cm (>75% effaced): 3

BISHOP SCORE

• Scoring (Cont.)– Cervical Consistency

• Firm cervical consistency: 0• Average cervical consistency: 1• Soft cervical consistency: 2

– Cervical Position• Posterior cervical position: 0• Middle or anterior cervical position: 1

– Zero Station Notation (presenting part level)• Presenting part at ischial spines -3 cm: 0• Presenting part at ischial spines -1 cm: 1• Presenting part at ischial spines +1 cm: 2• Presenting part at ischial spines +2 cm: 3

BISHOP SCORE• Modifiers

– Add 1 point to score for:• Preeclampsia• Each prior Vaginal Delivery

– Subtract 1 point from score for:• Postdates Pregnancy• Nulliparity• Premature or prolonged Rupture of Membranes

• Interpretation– Indications for Cervical Ripening with prostaglandins

• Bishop Score <5• Membranes intact• No regular contractions

– Indications for Labor Induction with Pitocin• Bishop Score >= 5• Rupture of Membranes


• Fetus (Cont.)– Palpation of Fetal Parts

• Fetal parts are felt earliest by 20 weeks of gestation• Disadvantages: Extremely subjective and subject to the skill

of examiner

– Auscultation of Fetal Heart Rate• Heard earliest by 18-20 weeks of gestation using ordinary

Pinard Stethescope• Heard earliest by 10 weeks of gestation using Doppler

Ultrasound• Disadvantages: Extremely subjective and subject to the skill

of examiner


• Fetus (Cont.)– Lightening

• Labour is likely to commence within 3 weeks of appearance of symptoms of lightening

• Disadvantages: Extremely subjective and can only be used to assess pregnancy in its later part

– Size of the Fetus• Change in uterine shape, volume of liquor amnii, hardening of the skull and

girth of the abdomen are of value in assessing he maturity of the fetus specially if the examinations are done by the same person at intervals

• Disadvantages: Extremely subjective and inaccurate in cases of Intrauterine Growth Restriction and fetuses which are Small for Gestational Age


INVESTIGATIONS

Recording of Positive

Pregnancy TestUltrasonography Radiography

INVESTIGATIONS• Recording of Positive Pregnancy Test

– The date of the first positive pregnancy test result allows the calculation of a minimum GA.

– This depends on the sensitivity of the test. – For example, if the test was performed 4 weeks ago and the

test is known to return positive results as early as 1 week after conception, then the minimum conceptional age (CA) would be 5 weeks (GA, 5 + 2 = 7 wk of amenorrhea)

• Radiography– Based on x-ray findings– Corresponds to the appearance and density of ossification

centres in the upper end of the tibia (38-40 weeks) and lower end of femur (36-37 weeks)

INVESTIGATIONS

• Ultrasonography– First Trimester: Crown-Rump Length (CRL) is most accurate

(Variation: ± 5 days)– Second Trimester: Biparietal Diameter (BPD), Head Circumference

(HC), (AC) and Femur Length (FL) is more accurate. This is best done between week 12 and 20. (Variation: ± 8 days)

– Third Trimester: Less reliable. (Variation: ± 16 days)– In clinical practice, when the difference between the gestational

age determined by sonographic measurement and the menstrual age is less than 10 days, the EDD is derived from the LNMP is confirmed. When the difference is more than 10 days, the EDD should be based on ultrasonographic fetal biometry

IMPORTANCE OF DETERMINATION OF GESTATIONAL AGE

• Accurate determination of gestational age is fundamental to obstetric care and is important in a variety of situations:– Antenatal test results will be inaccurate and misleading

when the dates are inaccurate– Fetal growth assessment, either clinically or by ultrasound

evaluation, relies on accurate assessment of gestational age.• Fetal growth retardation or macrosomia may be missed owing to

errors in gestational age assignment.

– Interpretation of antenatal biophysical testing subjected to variation with gestational age as well.

REFERENCE

• Konar H, D.C. Dutta’s Textbook of Obstetrics, 7th Edition, New Central Book Agency, 2010

• Mongelli M and Gardosi J, Update 19 April 2010, Evaluation of Gestation, Emedicine.medscape.com, Extracted from: http://emedicine.medscape.com/article/259269-overview

http://emedicine.medscape.com/article/259269-overview

http://emedicine.medscape.com/article/259269-overview

THE ENDThank You Very Much

gestational age

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