How Far Along Am I? Understanding Pregnancy Weeks and Gestational Age
DueDate.Baby Team
UK Pregnancy Experts
Wondering how far along you are in your pregnancy? Learn how pregnancy weeks are calculated, why midwives and the NHS count from your last period, and how to determine your gestational age accurately.
Quick Answer
Most pregnancies are measured from the first day of your last menstrual period (LMP), not from the day of conception. This means that when you are considered 4 weeks pregnant, conception likely occurred around 2 weeks earlier.
If you know the date of your last period, a due date calculator can estimate both how far along you are and your expected due date.
What Does "How Far Along Am I?" Mean?
When people ask how far along they are, they are asking about their gestational age. Gestational age is the number of weeks and days that have passed since the first day of your last menstrual period.
For example:
- 4 weeks pregnant: Approximately 4 weeks since your last period started.
- 12 weeks pregnant: Approximately 12 weeks since your last period started.
- 20 weeks pregnant: Halfway through a typical pregnancy.
- 40 weeks pregnant: Your estimated due date.
Maternity teams use gestational age because it provides a consistent way to track pregnancy development.
Why Pregnancy Is Counted From Your Last Period
Dating begins before conception because:
- Most people know when their last period began.
- Conception dates are often difficult to determine precisely.
- It provides a universal standard for midwives and clinicians.
In a typical 28-day cycle:
| Pregnancy Week | What Is Happening? | | :--- | :--- | | Week 1 | Menstrual period begins | | Week 2 | Ovulation approaches | | Week 3 | Conception may occur | | Week 4 | Pregnancy may be detected |
This means you are technically considered pregnant for around two weeks before fertilisation occurs.
How Can I Calculate How Far Along I Am?
Method 1: Use Your Last Menstrual Period
The simplest method is counting from the first day of your last period.
- Example: If your last period started 10 weeks ago, your gestational age is approximately 10 weeks.
Method 2: Use Your Due Date
If you already know your due date, work backwards. Because a full-term pregnancy is approximately 40 weeks:
- Due date in 20 weeks: Currently 20 weeks pregnant.
- Due date in 10 weeks: Currently 30 weeks pregnant.
Method 3: Use an Ultrasound Scan
If you are unsure of your dates, an early dating scan (usually between 10 and 14 weeks) provides the most reliable estimate by measuring the developing baby.
Common Pregnancy Milestones by Week
- 4 Weeks: Pregnancy test may become positive; implantation has recently occurred.
- 8 Weeks: Major organs are developing; baby is approximately the size of a raspberry.
- 12 Weeks: End of the first trimester; dating scan often takes place; risk of miscarriage decreases (read more in our First Trimester Roadmap).
- 20 Weeks: Anatomy scan is performed; many parents feel movement; halfway point (read more in our Bonding Guide).
- 28 Weeks: Third trimester begins; baby continues rapid growth.
- 40 Weeks: Estimated due date arrives.
Frequently Asked Questions
How many weeks pregnant am I from conception? To estimate gestational age from conception, add approximately two weeks to the time since fertilisation occurred.
Why does my pregnancy seem two weeks ahead? Because dating begins from your last period rather than conception, making gestational age approximately two weeks greater than the baby's actual developmental age.
Is gestational age the same as fetal age? No.
- Gestational age is measured from the last menstrual period.
- Fetal age is measured from conception (usually two weeks less).
What if my scan says a different number of weeks than I expected? This is common. Your midwife or sonographer may adjust your due date based on the dating scan if it provides a more accurate estimate.
Calculate Exactly How Far Along You Are
If you're unsure how many weeks pregnant you are, a due date calculator can provide an instant estimate based on your last menstrual period, conception date, or IVF transfer date.
Use the Duedate.baby Due Date Calculator to:
- Find out how far along you are.
- Discover your estimated due date.
- Track your pregnancy week by week.
Continue the Journey
Now that you know how pregnancy weeks are counted, explore the science and accuracy behind the numbers in How Accurate Is a Due Date Calculator?.