When Is The Neural Tube Formed In Pregnancy? | Week 3–4

The question “when is the neural tube formed in pregnancy?” comes up early for many parents. Here’s the plain answer, plus what it means for planning, screening, and folate intake.

When Is The Neural Tube Formed In Pregnancy? Timeline And Milestones

The neural tube begins as a flat plate of cells that folds and fuses into a tube. This happens fast. In humans, the head end (anterior neuropore) closes near day 25; the tail end (posterior neuropore) closes near day 28. That’s the fourth week after conception, or about the sixth week counting from the last menstrual period.

Stage	Approx. Day After Conception	What’s Happening
Neural plate appears	~18–19	Ectoderm thickens over the notochord; cells lengthen.
Neural groove forms	~20–21	Plate bends; raised folds flank a central groove.
Initial fusion points	~22–24	Folds meet and zip both forward and back.
Anterior neuropore closes	~25	Head end seals; brain vesicles begin shaping.
Posterior neuropore closes	~27–28	Tail end seals; primary neurulation completes.
Neural crest migration	~23–28	Border cells leave to form peripheral nerves and more.
Spinal canal forms	Late week 4	Tube separates from surface layer; lumen persists.

Why Timing Matters For Parents

This window is brief. By the time a home test turns positive, closure may be underway or finished. That’s why routine folic acid is a before-pregnancy habit, not just a prenatal fix. It’s also why early care, medication reviews, and temperature safety (no prolonged hot-tub heat) matter.

Neural Tube Formation In Early Pregnancy: Days And Risks

Development follows a tight schedule, and several factors can disrupt it. Low folate, certain anti-seizure medicines, uncontrolled diabetes, very high body mass index, and sustained maternal hyperthermia have all been linked with neural tube defects. Genetics can contribute too. Most families never learn a single cause, so prevention relies on stacking the odds with folic acid and smart early habits.

Folate Basics: What To Take And When

Folic acid helps the embryo synthesize DNA during rapid cell division. The common public-health advice is simple: take 400 micrograms daily if you can become pregnant, start at least a month before conception, and continue through the first trimester. If you had a prior neural tube defect in pregnancy, your clinician may advise 4,000 micrograms daily from one month before conception through 12 weeks, then switch to a standard prenatal dose.

For clear guidance on prevention and dosing, see the CDC folic acid guidance. For a global view on baseline dosing through 12 weeks, the WHO recommendation lays it out in one page.

Screening Windows That Catch Most Neural Tube Defects

Two standard checks cover this area. First, second-trimester maternal serum AFP between 15 and 22 weeks estimates risk for open neural tube defects. Second, the detailed anatomy ultrasound between 18 and 22 weeks reviews the skull, brain, spine, and skin cover. Abnormal results prompt a closer scan and, if needed, diagnostic testing.

What If Screening Is Positive?

Screen-positive AFP isn’t a diagnosis. It triggers confirmatory steps: re-date the pregnancy if needed, repeat blood work if timing was off, perform targeted ultrasound with a skilled sonographer, and your team may suggest amniotic-fluid testing when answers remain unclear. The care team will explain options and next steps at each stage.

How This Ties Back To Daily Life

The best time to act is before a missed period. Line up a prenatal vitamin with 400 micrograms of folic acid. If you use anti-seizure drugs, are living with diabetes, or had a prior neural tube defect in pregnancy, book a pre-conception visit and ask about higher-dose folic acid, tight glucose targets, and medication adjustments. Keep fevers down with provider-approved care, and skip long hot-tub sessions early on.

When Is The Neural Tube Formed In Pregnancy? Practical Q&A

How Does “Week” Counting Work?

Embryology counts days from fertilization; clinical obstetrics counts weeks from the last menstrual period. So “day 28 after conception” aligns with “about 6 weeks of gestation” on your chart. That’s why your ultrasound report may say “6w” when texts on embryology say “day 28.”

What Exactly “Closes” During Week 4?

Two openings called neuropores seal. The head end closes first, near day 25. The tail end follows by day 28. Failure at the head end causes anencephaly; failure at the tail end causes forms of spina bifida. Sealing is a zipper-like process that starts in the mid-back and moves in both directions.

Does Nutrition Beyond Folate Matter?

A balanced prenatal pattern helps overall growth. Folate is the headline here because the effect size for prevention is strong and the window is narrow. Iron, iodine, and vitamin D support other systems, but they don’t replace folic acid for neural tube protection.

Method Notes: Where These Numbers Come From

The closure days (about day 25 for the head end and day 28 for the tail end) are drawn from human embryology references and clinical reviews that map neuropore closure to Carnegie stages. The dosing ranges and timing for folic acid are distilled from national and global public-health guidance. Screening windows follow standard obstetric care pathways used in second-trimester AFP programs and the 18–22-week anatomy scan.

Takeaway You Can Use Today

Your action window opens before a positive test. Set 400 micrograms of folic acid as a daily habit now if pregnancy is possible, then keep regular care once you’re expecting. If your history includes a prior neural tube defect in pregnancy, ask about a temporary higher dose for the first trimester. And if screening flags a concern, know that ultrasound and follow-up testing can clarify what’s going on and guide care.

What Your Provider Looks For On Ultrasound

The mid-trimester anatomy scan reviews the skull bones, brain midline, ventricles, posterior fossa, and the shape of the head. It also tracks the spine in three planes to confirm the bony arches meet and the skin line is intact. Many open defects show a “lemon” or “banana” sign in the head, plus gaps in the spine. A normal study is reassuring, yet labs and dating still matter for AFP-based screens.

Primary And Secondary Neurulation, In Plain Language

Human embryos form the tube in two ways. The head, neck, and most of the trunk use a folding and zippering process called primary neurulation. The tail end forms by condensing a rod of cells that hollows out later, called secondary neurulation. Both routes meet and connect, building one continuous canal. That’s why the very last area to close is usually the lower back.

Medication And Supplement Review

Never stop a prescribed drug on your own, but do set a review before conception. Valproate and related agents can raise neural tube defect risk; many people can swap to options with lower risk under specialist care. Folate antagonists used for other conditions may also be an issue. On the supplement side, stay with folic acid unless your clinician gives a different plan. Product labels often list folate as “mcg DFE”; 400 mcg folic acid equals 667 mcg DFE on a US label.

Real-World Planning Checklist

Three Moves Before A Positive Test

• Pick a prenatal that lists 400 mcg folic acid.
• Set a reminder so the dose never gets missed.
• Book a check-in if you take anti-seizure drugs or manage diabetes.

First Six Weeks After The Last Period

• Keep the folic acid dose steady through week 12.
• Skip long soaks in very hot water; treat fevers promptly.
• Set your prenatal visit and ask about AFP timing.

Weeks 15–22

• Complete the lab screen (quad screen or AFP) if offered.
• Schedule the 18–22-week anatomy scan at a quality center.
• If a result is off, expect a targeted scan and a calm stepwise plan.

Common Terms Without The Jargon

Neural Plate

The early sheet of cells that will fold into the tube.

Neuropore

The temporary opening at each end of the tube; one at the head, one at the tail.

Spina Bifida (Open)

A gap in the spine and skin from failed closure at the tail end; can vary in size and level.

Anencephaly

A severe defect when the head end fails to close; pregnancies with this finding need careful counseling.

AFP (Alpha-fetoprotein)

A protein from the fetus that rises in maternal blood with open neural tube defects; used as a screen in the second trimester.

Answering The Core Question, One More Time

When friends ask, “when is the neural tube formed in pregnancy?” the most practical reply is week 3 to week 4 after conception, with closure by about day 28. Those dates translate to roughly the sixth week on an obstetric calendar. That’s why habits in the month before conception matter so much.

Risk Snapshot And What You Can Control

Most pregnancies are unaffected. For those that are, risk often reflects a mix of background chance, genes, and environment. Here’s a quick control-panel view of steps within reach.

Situation	Action To Discuss	Why It Helps
Planning pregnancy	Start 400 mcg folic acid daily	Builds folate stores before week-4 closure
Past NTD-affected pregnancy	Ask about 4,000 mcg until 12 weeks	Lowers recurrence risk in high-risk settings
Using valproate or similar	Review meds with neurology/OB	Some agents raise NTD risk; alternatives exist
Diabetes or very high BMI	Pre-pregnancy tune-up	Better glucose and weight lower defect risk
Frequent hot-tub use	Skip sustained high heat early	Reduces prolonged hyperthermia exposure
Fever in early weeks	Call about treatment fast	Managing temperature is safer for the embryo
Folate-poor diet	Use a prenatal plus fortified grains	Reaches the 400 mcg target reliably