Dating of the pregnancy

Dating of the pregnancy

Dating the pregnancy refers to:

• Determining the gestational age of pregnancy
• Determining the expected date of delivery
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Accurate dating is important because:

• It guides antenatal care appointments
• Guides timing of investigations
• Allows proper assessment of fetal growth
• Helps in proper management of preterm birth or post-term pregnancy

Incorrect dating might lead to inappropriate obstetric interventions.

Stages of gestational age:

** first trimester: 0-13 weeks

** second trimester: 14-27 weeks

** third trimester: 28-40 weeks

Pregnancy is 40 weeks (280 days) calculated from the first day of the last menstrual period.

1. Last menstrual period (LMP):

• The most commonly used method to date a pregnancy
• We use the first day of the last menstrual period
• The expected due date is calculated using Naegele’s rule which depends on the last menstrual period
• It is simple to use
• Has some limitations like irregular cycles, uncertain LMP, breastfeeding or using OCPs

2. Using ultrasound for dating:

• The most accurate method
• In the first trimester we measure crown-rump length
• In the second trimester we measure head circumference
• Used if LMP unknown, irregular cycles or if there was a discrepancy between calculated dates and fetal measurements
• Cany be used to change dates in the third trimester (increased margin of error)

3. Clinical examination for dating:

• Abdominal exam to estimate uterine size
• Fundal height measurement
• Maternal feeing of fetal quickening
• This is the least accurate method

It is best to date pregnancy as early as possible, because with advancing gestations the margin of error of different dating methods increases which leads to poor maternal and fetal outcomes should intervention be needed.

Dr Najeeb Layyous  F.R.C.O.G

Consultant Obstetrician, Gynecologist and Infertility Specialist