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Week 33weeks
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First Trimester Guide — Pregnancy Weeks 1 to 12
Navigate your first trimester (weeks 1-12). Learn about implantation, fetal development milestones, morning sickness, and essential prenatal care.
Interactive Calculator
Your Pregnancy Details
First day of your last period. Gestational age is calculated from this date.
Your Pregnancy Timeline
Trimester
Weeks 1–13
~38 weeks
Days Remaining
266days
Week 15% completeWeek 40
0
4
8
12
16
20
24
28
32
36
40
Weeks 1–13
Major organs and systems form. Common symptoms include fatigue, nausea, and breast tenderness.
Tri 2
Second Trimester
Weeks 14–27
Baby grows rapidly. Movement felt. Symptoms often improve. Anatomy scan at 18–22 weeks.
Tri 3
Third Trimester
Weeks 28–40+
Baby continues maturing. Braxton Hicks, back pain, and nesting instinct are common.
Pregnancy Milestones
✓
Conception
Week 0
✓
Embryo Implants
Week 2
6
Heartbeat Detectable
Week 6
12
NT Scan
Week 12
20
Anomaly Scan
Week 20
24
Viability Threshold
Week 24
28
Third Trimester Begins
Week 28
37
Full Term
Week 37
40
Due Date
Week 40
You are in the First Trimester (Weeks 1–13). You are week 3, day 0 of your pregnancy. Approximately 266 days (38 weeks) remain until your due date.
The formula
The formula
What Gestational Age Measures
Pregnancy is measured in gestational age, which counts from the first day of your last menstrual period (LMP), not from the date of conception. This is the standard method used by obstetricians and midwives worldwide because the LMP date is typically the most reliably remembered reference point. A full-term pregnancy is defined as 40 weeks (280 days) from LMP, though only about 5% of babies are born on their exact due date.
Gestational age is the universal reference for scheduling prenatal tests, assessing fetal development milestones, determining viability, and planning delivery. Understanding your current pregnancy week helps you know what to expect at each stage of your pregnancy journey and ensures timely prenatal care.
The Gestational Age Formulas
Days Since LMP = Today − LMP
Current Week = Floor(Days Since LMP / 7) + 1
Current Day = Days Since LMP mod 7
Days Until Due Date = 280 − Days Since LMP
LMP = first day of last menstrual periodFloor = round down to nearest integer
Formula Source
This calculator uses the **standard gestational age calculation (days from LMP to today converted to weeks)** from **ACOG (American College of Obstetricians and Gynecologists)** guidelines.
Reference URL: https://www.acog.org/womens-health/infographics/pregnancy-timeline
Last Verified: 2026-07-30
Worked Example
If your LMP was January 1, 2026 and today is August 9, 2026: Days since LMP = 220 days. Current Week = Floor(220 / 7) + 1 = Floor(31.4) + 1 = 31 + 1 = 32. Current Day = 220 mod 7 = 3. This means you are 32 weeks and 3 days pregnant. Days until due date = 280 − 220 = 60 days remaining (approximately 8 weeks and 4 days). Your due date calculated from Naegele's Rule (LMP + 280 days) would be October 8, 2026.
Trimester Breakdown
| Trimester | Weeks | Key Developments |
|---|---|---|
| First Trimester | Weeks 1–13 | Organ formation, heartbeat detectable, neural tube closure |
| Second Trimester | Weeks 14–27 | Growth, fetal movement felt, anatomy scan, viability threshold |
| Third Trimester | Weeks 28–40+ | Rapid weight gain, lung maturation, head-down positioning, labor preparation |
FAQ-Style Explanations
What is the difference between gestational age and fetal age? Gestational age is measured from LMP (about 40 weeks total) and is the standard medical reference. Fetal age (or conceptual age) is measured from conception and is about 2 weeks less than gestational age. This calculator uses gestational age, which is what your healthcare provider will use.
What if I have irregular cycles? For people with regular 28-day cycles, LMP-based calculation is reasonably accurate. However, if you have irregular cycles or are unsure of your LMP, first-trimester ultrasound dating is more reliable. Crown-rump length (CRL) measurement between 7–13 weeks is considered the most accurate method for dating a pregnancy, with an error margin of only ±3–5 days.
Is it normal for my calculated weeks to differ from my ultrasound? Yes, it is common for LMP-based calculations to differ from ultrasound dating. If the difference is more than 5–7 days in the first trimester, most providers will use the ultrasound date as the reference. Discrepancies often arise from irregular ovulation timing or inaccurate LMP recall.
Known Limitations
- Assumes a regular 28-day menstrual cycle with ovulation on day 14. Cycles shorter or longer than 28 days will shift the actual ovulation date and therefore the true gestational age.
- Relies on accurate recall of the LMP date, which may not be reliable for all individuals.
- Does not account for variations in the luteal phase length, which can range from 10–16 days across different individuals.
- For IVF pregnancies, the gestational age should be calculated from the transfer date and embryo age, not from LMP.
- The 40-week (280-day) standard is an average — normal pregnancies range from 37 to 42 weeks. Post-term pregnancies beyond 42 weeks may require medical intervention.
- First-trimester ultrasound dating is considered more accurate than LMP-based calculation and is preferred when available, especially for clinical decision-making.
Scenario guide
Scenario guide
Weeks 1 to 4 — Fertilization to Implantation
Weeks 1 and 2 of pregnancy are technically counted as part of gestation even though pregnancy has not yet occurred — these correspond to the menstrual period and the pre-ovulation follicular phase. At week 3, ovulation happens and the egg is fertilized by sperm in the fallopian tube. The resulting zygote divides as it travels toward the uterus, becoming a blastocyst by day 5 to 6. Implantation into the uterine lining typically occurs around day 6 to 10 after fertilization, and at this point human chorionic gonadotropin (hCG) production begins. By the end of week 4 (LMP week 4), hCG levels are high enough to be detected by most home pregnancy tests, which typically have a sensitivity of 20 to 25 mIU/mL.
Weeks 5 to 8 — Organogenesis
Weeks 5 through 8 are the most critical period for fetal development because virtually all major organs and systems are formed during this window, a process called organogenesis. By week 5, the gestational sac is visible on transvaginal ultrasound, and the yolk sac can be seen inside it. The fetal heartbeat becomes visible by Doppler ultrasound around week 6, beating at approximately 90 to 110 beats per minute. By week 8, the embryo measures about 1.6 cm (0.6 inches) from crown to rump and has developed all major organ systems in rudimentary form, including the brain, heart, lungs, liver, and kidneys. This period is also when exposure to teratogens — substances that can cause birth defects — is most damaging, which is why prenatal vitamins with folic acid (400 to 800 mcg daily) are recommended before conception and throughout this window.
Weeks 9 to 12 — Fetal Features Form
By week 9, the embryo is officially called a fetus, and the period of highest vulnerability to organ malformation is largely over. Between weeks 9 and 12, facial features become more defined, fingers and toes separate fully, and the fetus begins making involuntary movements that are not yet felt by the mother. The miscarriage risk, which was approximately 10% in the earliest weeks, drops to about 3% by week 12 and falls below 1% after the 12-week mark. The nuchal translucency (NT) scan, performed between weeks 11 and 14, measures the fluid collection at the back of the fetal neck and screens for chromosomal abnormalities such as Down syndrome. By the end of week 12, the fetus is about 5.4 cm (2.1 inches) long and weighs approximately 14 grams (0.5 ounces).
Morning Sickness Pattern
Morning sickness, more accurately described as nausea and vomiting of pregnancy, affects an estimated 70% to 80% of pregnant individuals. It typically begins around week 6, peaks at week 9, and resolves for most people by weeks 14 to 16. The condition is linked to the rapid rise in hCG and estrogen levels during early pregnancy. While common and usually benign, severe persistent vomiting that prevents adequate hydration is called hyperemesis gravidarum, which affects approximately 0.3% to 3% of pregnancies and may require medical intervention including intravenous fluids and antiemetic medication. Eating small frequent meals, ginger, vitamin B6 (pyridoxine), and doxylamine are first-line treatments recommended by ACOG.
Key Prenatal Tests and Visits
The first prenatal visit typically occurs between weeks 8 and 12 and includes a comprehensive medical history, physical exam, blood work (including blood type, Rh factor, CBC, rubella and varicella immunity, HIV, hepatitis B, and syphilis screening), and a urine test. ACOG recommends that all pregnant individuals take a daily prenatal vitamin with at least 400 mcg of folic acid starting before conception through the first trimester to reduce the risk of neural tube defects by up to 70%. The first-trimester combined screening, performed between weeks 11 and 14, includes the nuchal translucency ultrasound measurement along with maternal serum PAPP-A and hCG levels to estimate the risk of chromosomal abnormalities. An early dating ultrasound between weeks 8 and 13 provides the most accurate gestational age estimate and is especially valuable for women with irregular cycles or uncertain LMP dates.
FAQ
Frequently Asked Questions
The fetal heart begins beating around week 6 of pregnancy (LMP), beating at approximately 90 to 110 beats per minute. It can typically be visualized by transvaginal ultrasound at week 6 and becomes audible by Doppler device around week 10 to 12. The heartbeat rate gradually increases during the first trimester, reaching 170 to 190 beats per minute around week 9 before settling to 120 to 160 beats per minute for the remainder of pregnancy. If no heartbeat is seen at the expected gestational age, your provider may schedule a follow-up ultrasound to confirm dating.
The overall miscarriage rate in the first trimester is approximately 10%, with the highest risk in the earliest weeks. You can reduce risk by taking a prenatal vitamin with at least 400 mcg of folic acid daily starting before conception, avoiding alcohol, tobacco, and recreational drugs, limiting caffeine to less than 200 mg per day, managing chronic conditions like diabetes and thyroid disease, and avoiding exposure to known teratogens. ACOG notes that moderate exercise and normal daily activity are safe and do not increase miscarriage risk. If you experience heavy bleeding or severe cramping, contact your provider immediately.
Common first trimester symptoms include morning sickness (nausea and vomiting, affecting 70 to 80% of pregnant individuals), fatigue, breast tenderness and enlargement, frequent urination, food aversions or cravings, mood swings, constipation, and a heightened sense of smell. Morning sickness typically peaks around week 9 and improves by weeks 14 to 16. Fatigue is most pronounced in the first trimester due to surging progesterone levels and the body diverting energy toward building the placenta. Most of these symptoms are normal and resolve as the pregnancy progresses, but contact your provider if symptoms become severe or are accompanied by fever, heavy bleeding, or severe abdominal pain.
ACOG recommends scheduling your first prenatal visit as soon as you confirm a positive pregnancy test, ideally between weeks 8 and 12. The first visit includes a comprehensive medical history, physical examination, blood work (blood type, Rh factor, CBC, immunity screening for rubella and varicella, HIV, hepatitis B, and syphilis testing), and a urine analysis. An early dating ultrasound between weeks 8 and 13 can confirm gestational age and is especially helpful for women with irregular cycles. The nuchal translucency scan, part of first-trimester combined screening, is performed between weeks 11 and 14.
The nuchal translucency (NT) scan is an ultrasound measurement of the fluid-filled space at the back of the fetal neck, performed between weeks 11 and 14 of pregnancy. Increased nuchal translucency is associated with a higher risk of chromosomal abnormalities such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). The NT measurement is combined with maternal serum levels of PAPP-A and hCG in what is called the first-trimester combined screening, which can detect approximately 82 to 87% of Down syndrome cases. An abnormal screening result does not confirm a diagnosis and is typically followed by diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis.
Keep going
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