Week-by-Week Pregnancy Guide: What to Expect from Week 1 to Week 40 (2026 Complete Guide)

TL;DR

A full-term pregnancy lasts about 40 weeks, counted from the first day of your last menstrual period (LMP), and is divided into three trimesters (NHS, Mayo Clinic). The first trimester (weeks 1 to 12) is when the baby's major organs form and morning sickness is common. The second trimester (weeks 13 to 27) is often the most comfortable, with the bump showing and the baby's first movements felt. The third trimester (weeks 28 to 40) is about rapid growth and preparing for birth. Your baby grows from the size of a poppy seed (week 4) to a small watermelon or pumpkin (week 40), reaching about 3 to 3.5 kg. Attend all antenatal checkups, eat well, stay active, and report any warning signs promptly.

Quick Answer

Pregnancy lasts about 40 weeks from your last period, split into three trimesters. Weeks 1 to 12: organs form, morning sickness is common. Weeks 13 to 27: the bump shows and you feel movements. Weeks 28 to 40: rapid growth and birth preparation. The baby grows from a poppy seed to about 3 to 3.5 kg.

Author: Mylo Editorial Team, Mylo Parenting Desk Medically reviewed by: Mylo Editorial Board, aligned with NHS, Mayo Clinic, ACOG and FOGSI guidance Last updated: 11 June 2026

Medical Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice. Every pregnancy is different, and the sizes, dates and symptoms here are averages. Always attend your antenatal appointments and contact your doctor with any concerns or warning signs.

Key Takeaways

• Pregnancy is counted as 40 weeks from the first day of your last period (LMP) (NHS)

• It is divided into 3 trimesters: weeks 1 to 12, 13 to 27, and 28 to 40

• First trimester: organs form, miscarriage risk is highest, nausea common

• Second trimester: most comfortable, bump shows, movements felt (around weeks 18 to 22)

• Third trimester: rapid growth, baby gains most weight, birth preparation

• The baby grows from a poppy seed to about 3 to 3.5 kg at full term

• Full term is 39 to 40 weeks; due date is an estimate, not a fixed day

• Attend all antenatal checkups and report warning signs promptly

How Is Pregnancy Measured in Weeks?

Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception (Mayo Clinic). This means:

• Weeks 1 and 2: technically before conception (your body is preparing to ovulate)

• Conception usually happens around week 2 to 3

• A full-term pregnancy is about 40 weeks (around 280 days)

• Due date is estimated as 40 weeks from your LMP, but only about 1 in 20 babies arrive exactly on it

The Three Trimesters

First Trimester: Weeks 1 to 12 (Organs Form)

This is the most critical period of development, when the baby's major organs take shape (NHS, Cleveland Clinic):

First trimester focus: Confirm pregnancy, start antenatal care, eat folate-rich foods (and any doctor-advised supplement), avoid alcohol and smoking, and rest. The first scan (dating scan) usually happens between weeks 8 and 12 (NHS).

Second Trimester: Weeks 13 to 27 (Growth and Movement)

Often the most comfortable trimester, nausea eases and energy returns (NHS, Mayo Clinic):

Second trimester focus: The important anomaly scan (around week 20) checks the baby's development. Stay active, eat iron- and calcium-rich foods, and start thinking about birth plans (ACOG).

Third Trimester: Weeks 28 to 40 (Rapid Growth and Birth Prep)

The baby gains most of its weight and gets ready for birth (NHS, Cleveland Clinic):

Third trimester focus: Watch for labour signs, attend frequent checkups, prepare your hospital bag (by week 34), and learn the difference between Braxton Hicks and real labour. Full term is 39 to 40 weeks (ACOG).

What Are the Key Pregnancy Milestones by Week?

What Should You Do in Each Trimester?

(ACOG, NHS)

When Should You Call Your Doctor During Pregnancy?

Contact your doctor promptly at any stage if you have (NHS, ACOG):

• Vaginal bleeding (any amount)

• Severe or persistent abdominal or pelvic pain

• Severe headache, blurred vision or sudden swelling (possible pre-eclampsia)

• Reduced or absent baby movements (especially in the third trimester)

• Fluid leaking from the vagina (possible water breaking)

• Regular contractions before 37 weeks (possible preterm labour)

• High fever

• Painful or burning urination

• Severe vomiting that prevents keeping food or water down

• Any symptom that worries you, it is always okay to get checked

Indian Context: What Indian Mothers Should Know

• Antenatal visits: Follow the recommended schedule; government schemes (JSY, PMSMA) support free antenatal care and delivery

• Nutrition: Include dal, palak, methi, ragi, dairy, fruits and any doctor-advised supplements; anaemia is common in Indian women (ICMR)

• No gender determination: Sex determination is illegal in India (PCPNDT Act); scans are for health, not gender

• Hydration in heat: Drink water, coconut water and chaas, especially in summer

• Vaccination: Take the TT or Tdap and other advised vaccines on schedule

• Hospital planning: In tier-2 and tier-3 cities, plan your delivery hospital and transport in advance; keep the bag ready by week 34

• Emergency number: Dial 108 for ambulance services across most states

Myths vs Facts About Pregnancy Weeks

FAQs: Week-by-Week Pregnancy

How many weeks is a full pregnancy? A full-term pregnancy is about 40 weeks, counted from the first day of your last period (NHS). Full term is considered 39 to 40 weeks, while 37 to 38 weeks is "early term" and after 42 weeks is "post-term."

Pregnancy kitne hafte ki hoti hai? (Hinglish) Poori pregnancy lagbhag 40 hafte (around 9 mahine plus 1 hafta) ki hoti hai, jo aapke last period ke pehle din se gini jati hai. Ise 3 trimesters mein baanta gaya hai: pehla (1 se 12 hafte), doosra (13 se 27 hafte), aur teesra (28 se 40 hafte). Full term 39 se 40 hafte mana jata hai.

When can I feel my baby move? Most women feel the first movements (called quickening) between weeks 18 and 22 (NHS). First-time mothers often feel them later than mothers who have been pregnant before. By the third trimester, movements become strong and regular.

Which trimester is the most critical? The first trimester (weeks 1 to 12) is the most critical for development, as the baby's major organs form and the risk of miscarriage is highest (Cleveland Clinic). This is why avoiding alcohol, smoking and unsafe medicines is especially important early on.

Pregnancy ke kis hafte mein scan hota hai? (Hinglish) Pehla (dating) scan aksar 8 se 12 hafte ke beech hota hai. Bahut important anomaly scan 18 se 21 hafte (around 20 weeks) mein hota hai, jo baby ke development ko check karta hai. Teesre trimester mein growth scan ho sakte hain. Yaad rakhein, India mein gender jaanna gairkanooni hai (PCPNDT Act).

How big is the baby at each stage? The baby grows from a poppy seed at week 4, to an avocado at week 16, a banana at week 20, an eggplant at week 28, and finally a small watermelon or pumpkin (about 3 to 3.5 kg) at week 40 (Mayo Clinic). These are averages; healthy babies vary in size.

What is the viability week? Around week 24, the baby reaches a developmental milestone where survival outside the womb becomes possible with intensive medical care (NHS). Survival and health outcomes improve significantly with each additional week in the womb.

When is the baby considered full term? A baby is considered full term at 39 to 40 weeks (ACOG). Babies born at 37 to 38 weeks are "early term," and those born before 37 weeks are "preterm." Staying pregnant to full term gives the lungs and brain more time to mature.

Is my due date exact? No, it is an estimate. Only about 1 in 20 babies arrive exactly on the due date (Mayo Clinic). Most babies are born within 1 to 2 weeks before or after it. Your doctor may adjust the date based on early ultrasound measurements.

How much weight should I gain during pregnancy? It depends on your pre-pregnancy BMI, but most women gain about 10 to 16 kg total, with very little in the first trimester and most in the second and third (ACOG). Your doctor will give you a personalised target.

When should I start counting baby movements? From around 28 weeks, pay attention to your baby's pattern of movements (NHS). If you notice a clear reduction or change in movements, contact your doctor or maternity unit immediately, do not wait.

References

1. NHS UK. "Pregnancy Week by Week." https://www.nhs.uk/pregnancy/week-by-week/

2. NHS UK. "Your Pregnancy Care (antenatal care, checks and screening tests)." https://www.nhs.uk/pregnancy/your-pregnancy-care/

3. Mayo Clinic. "Fetal Development: First and Second Trimester." https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development/art-20045302

4. Cleveland Clinic. "Fetal Development." https://my.clevelandclinic.org/health/articles/fetal-development

5. American College of Obstetricians and Gynecologists (ACOG). "How to Tell When Labor Begins." https://www.acog.org/womens-health/faqs/how-to-tell-when-labor-begins

6. ACOG. "Nutrition During Pregnancy." https://www.acog.org/womens-health/faqs/nutrition-during-pregnancy

7. ACOG. "Weight Gain During Pregnancy." https://www.acog.org/womens-health/faqs/weight-gain-during-pregnancy

8. ICMR / NIN. "Dietary Guidelines for Indians." https://www.nin.res.in/dietaryguidelines/pdfjs/locale/DGI07052024P.pdf

9. FOGSI (Federation of Obstetric and Gynaecological Societies of India). https://www.fogsi.org/