Dating Your Pregnancy: Why Accurate Due Dates Matter

In short: Pregnancy lasts 40 weeks (280 days), counted from the first day of the last period. The most common way to date a pregnancy is from the last period (Naegele’s rule); the most accurate is ultrasound, especially in the first trimester by measuring the baby’s length. Accurate dating matters for following the baby’s growth and managing preterm or overdue birth.

What Does “Dating the Pregnancy” Mean?

Dating the pregnancy refers to:

• Determining the gestational age of pregnancy
• Determining the expected date of delivery

Why Accurate Dating Matters for You and Your Baby

• 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, which is why getting it right matters from the very first prenatal visit.

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.

How Is Pregnancy Dated?

• 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
• 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
• Can be used to change dates in the third trimester (increased margin of error)
• Clinical examination for dating:
• Abdominal exam to estimate uterine size
• Fundal height measurement
• Maternal feeling of fetal quickening
• This is the least accurate method

It is best to date your pregnancy as early as possible, because with advancing gestation the margin of error of different dating methods increases, which can lead to poorer maternal and fetal outcomes should intervention be needed. If you’re pregnant or planning a pregnancy and would like a thorough, early assessment, Dr. Layyous and his team are here to guide you through every stage of your antenatal care.

Dr Najeeb Layyous F.R.C.O.G

Consultant Obstetrician, Gynecologist and Infertility Specialist