By Dr Tan Shu Qi, Consultant and Associate Professor Tan Thiam Chye, Visiting Consultant, Department of Obstetrics and Gynaecology, KK Women's and Children's Hospital

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After finding out that you're pregnant, the next big question is: When will your baby make an appearance? Here are some easy ways you can calculate the estimated date of delivery (EDD):

1. Count Down with Naegle's Rule

count down with naegle's rule

If you have a regular 28-day period cycle, simply count the days to your EDD from the first day of your last menstrual period (LMP). Although your baby was conceived about 14 days after your last period started, you'll need to count 40 weeks or 280 days down from the first day of your LMP.

Put simply, add seven days to the first day of your last menstrual period, and nine months to that date.

It's important to note that babies don't usually arrive strictly on time. Your baby may come any time after 37 weeks of pregnancy, also known as term. However, some pregnancies may continue till week 42. The reference for EDD is at week 40.

Naegle's Rule only applies if you have regular cycles, which don't necessarily need to be a regular 28-day menstrual cycle. Just adjust the calculation accordingly. If you have a 21-day or 35-day cycle, simply subtract or add seven days from/to the equation.

Related: Week 1: Your Last Period Is Worth Remembering

2. Use a Pregnancy Calendar Wheel or an Online Calculator

use a pregnancy calendar wheel or an online calculator

Alternatively, search for a pregnancy calendar wheel or a pregnancy calculator online. Just type in the dates or rotate the wheel to line the date of your LMP with the indicators — and you'll be able to see your EDD immediately. It doesn't get easier than that!

3. Schedule an Ultrasound Scan

schedule an ultrasound scan

A first trimester ultrasound scan is the most accurate dating method, which is usually scheduled when you are eight to 12 weeks pregnant. Your doctor will be able to give you an accurate due date then. So don't worry if you have irregular periods or you can't recall your LMP.

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Sources:

The New Art and Science of Pregnancy and Childbirth 2008, World Scientific

Healthy Start for your Pregnancy 2012, Health Promotion Board Singapore