What week am I in? How to calculate your BF or estimated due date
Verified and reviewed by: Midwife Maria Midstam
Knowing when your baby is likely to be born can provide both security and control during pregnancy. That date is called BF - expected birth, and is used as a guide for you and your midwife throughout your pregnancy.
What does BF - expected birth mean?
BF stands for estimated time of delivery and marks the date when the pregnancy reaches 40 full weeks (280 days) from the first day of your last period.
This is the date the healthcare system uses to:
- Monitoring the child's growth and development
- Plan checks and possible investigations
- Assessing if the pregnancy is overdue or starts too early
Quick response:
BF is an estimate, not an exact date. Only about 4-5 % of all babies are born on their estimated date - most are born within two weeks before or after.
How to calculate your BF - step by step
The most common way to calculate BF is to start from the first day of your last period.
- Note the first day of your last period.
- Please add 280 days (or 40 weeks).
- The date you get is yours estimated date of birth (ED).
Example:
If your last period started on January 1, yours will be estimated date of delivery October 8.
Don't want to do the math yourself? Use a pregnancy calculator - it automatically calculates your BF based on your menstrual cycle.
Other ways to determine BF
Every pregnancy is unique, and sometimes more precise methods than the calendar are needed.
Ultrasound in early pregnancy
An early ultrasound measures the length of the fetus. Based on this, the midwife assesses length of pregnancy and expected date of delivery.
Routine ultrasound (RUL) - week 18-20
During RUL, the fetal development, placenta and amniotic fluid volume are checked. If the measurements show that the baby is older or younger than expected, the BF can be adjusted.
In the case of planned childbirth
For example planned caesarean section or delivery a precise delivery date is set based on medical assessment.
Quick response:
The BF can be adjusted if ultrasound shows a different gestational age than that calculated from the menstrual date.
Things to consider about your BF
BF is a estimate - not a guarantee.
The baby will come when both the body and the baby are ready.
- Your midwife will monitor the progress and may adjust the date.
- Most births start between weeks 38 and 42.
- Don't get stressed if the date passes - it's perfectly normal.
If you have passed BF without signs of labor, you are often booked additional controls to see that everything looks fine - it's standard and nothing dangerous.
Example: how BF is used in healthcare
- At the time of enrollment with the midwife, checks are planned based on the BF.
- At the RUL in weeks 18-20, the date is adjusted if necessary.
- In the case of overweight (after week 42), a possible start-up is planned.
FAQ - Frequently asked questions about BF (expected birth)
What does EDD mean?
BF stands for estimated date of delivery - the date the pregnancy reaches 40 weeks.
How to calculate BF?
Take the first day of your last period and add 280 days (40 weeks).
Is BF an exact date?
No. Only about 5 % give birth on their BF date, most ±2 weeks.
When can the BF be changed?
If ultrasound shows a different gestational age or if you have irregular periods.
What happens at the routine ultrasound?
In weeks 18-20, fetal development is checked and the BF can be adjusted.
Can you calculate BF without a menstrual date?
Yes, by ultrasound in early pregnancy.
What is the average length of pregnancy in Sweden?
Approximately 280 days - that is, 40 weeks from the first day of your last period.
To understand when your baby is due to be born can provide both security and control during pregnancy. That date is called BF (expected birth) and is an estimate of when the birth is likely to take place.
Read also: Guide: how to take out pregnancy insurance
