Embryology & Early Development
First trimester
The first trimester is the first third of pregnancy, roughly weeks 1 through 12, when the neural tube forms and closes.
In short: First trimester
- Neural tube defects originate in the first trimester.
- Early ultrasound can sometimes detect encephalocele in this window.
- Folic acid is especially important before and during this time.
- It covers the earliest weeks of pregnancy.
What it is
It is the first third of pregnancy, roughly weeks 1–12, during which the neural tube forms and closes. It is the period when the foundations of the nervous system are laid.
Understanding First trimester
Because the neural tube closes in the first trimester, this is when encephalocele originates, and early ultrasound can sometimes detect it toward the end of this window (around 11–13 weeks). Folic acid is especially important before and during this time, which is why it is recommended starting before conception.
Why it matters
Understanding the first trimester helps families see why folic acid is emphasized so early and why some encephaloceles can be detected by late first-trimester ultrasound.
Common questions
When is the first trimester?
Roughly weeks 1 through 12 of pregnancy.
What happens then?
The neural tube forms and closes.
Why is folic acid important then?
It helps reduce the risk of some neural tube defects.
Bottom line
The first trimester is when the neural tube forms and closes, and encephalocele originates.
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