Pregnancy Calendar

How is the due date calculated?

Calculating the due date relies on a simple but universally used method: Naegele's rule. This involves adding 280 days (40 weeks) to the date of the first day of the last menstrual period.

Why this basis? Because on average, a pregnancy lasts 40 weeks of amenorrhea (SA), which is about 38 weeks of actual pregnancy after fertilization.

However, this calculation remains an estimate :

• Some women ovulate earlier or later.
  
• Cycles can be shorter or longer than 28 days.
  
• Implantation may vary by a few days.
  

Our pregnancy calendar provides you with a probable due date , but only a healthcare professional can confirm it during the first trimester ultrasound.

2. The stages of pregnancy week by week

Pregnancy progresses dramatically, and each week brings its share of changes for both mother and baby. Here is a clear and helpful summary:

Weeks 1 to 4 – The beginning of the adventure

Implantation, the beginning of placental formation, and the rise in hormones. The first symptoms may appear.

Weeks 5 to 12 – Organ Formation

This is the period when the heart, brain, and vital organs develop. The first trimester often ends with a dating ultrasound.

Weeks 13 to 27 – The 2nd trimester

The baby moves, facial features develop, and the expectant mother often regains energy. Many consider this trimester the most comfortable.

Weeks 28 to 40 – The final sprint

Growth accelerates, the baby gains weight and positions itself for birth. The body gradually prepares for delivery.

3. What are the first signs of pregnancy?

Even before a positive test, the body sometimes sends some signals:

• Intense fatigue
  
• Sensitive breasts
  
• Morning sickness
  
• Mood swings
  
• Slight twinges in the lower abdomen
  
• Altered food tastes
  
• More intense odors
  
• Minor implantation bleeding
  
• Delayed period
  

These signs are not enough to confirm a pregnancy, but they are very common.

Only a urine or blood test can officially confirm the start of pregnancy.

4. Calculation of weeks of amenorrhea (WA) and weeks of pregnancy (WG)

There are two ways to count a pregnancy, which can cause confusion:

• Weeks of amenorrhea (SA) : the medical standard, calculated from the first day of the last period.
  
• Weeks of pregnancy (WG) : approximately 2 weeks less than weeks of gestation (WG), because they begin at fertilization.
  

Example :

10 SA = approximately 8 SG.

Our calendar uses the calculation in SA , because that is the one used by doctors, midwives and official ultrasounds.

5. Mandatory ultrasounds during pregnancy

Three ultrasound scans structure the follow-up care in France:

First ultrasound (T1) – Between 11 and 13 weeks of gestation

Precise dating, nuchal translucency measurement, verification of overall development.

2nd ultrasound (T2) – Around 22 weeks of gestation

Complete morphological analysis: heart, kidneys, limbs, brain, placenta, baby's position.

3rd ultrasound (T3) – Around 32 weeks of gestation

Growth monitoring, weight estimation, amniotic fluid assessment and final position.

These steps are essential to check the baby's good health and anticipate any special needs.

6. Weight, BMI and pregnancy: how much to take?

The ideal weight gain varies depending on the BMI before pregnancy:

• Low BMI (<18.5) → +12 to +18 kg
  
• Normal BMI (18.5–24.9) → +11 to +16 kg
  
• Overweight (25–29.9) → +7 to +11 kg
  
• Obesity (≥30) → +5 to +9 kg
  

Each body develops differently: the distribution of weight (baby, placenta, blood, fluid…) makes this weight gain normal and necessary.

Medical supervision helps to avoid taking too little or too much.

If you want to calculate your BMI, we have created a free tool that you can use in 20 seconds.

>>> BMI Calculator (free) 📝

7. Calculating the weeks remaining until the due date

By entering the date of your last period, our calculator will immediately tell you:

• your current stage of pregnancy (e.g., 18 weeks pregnant → 4th month)
  
• the number of weeks remaining
  
• the estimated due date
  

Practical for planning your appointments, preparing your maternity bag and tracking your baby's development.

8. Reliability of the expected delivery date

The due date (EDD) remains an estimate , as only 5% of babies are born on that exact day.

It can vary depending on:

• your cycle
  
• your ovulation
  
• fertilization
  
• baby's growth
  

The date becomes truly reliable after the dating ultrasound (T1), where the crown-rump length (CRL) is measured.

9. Tips for living well each quarter

A balanced pregnancy requires simple, good habits:

• Eat regular meals
  
• Sleep as much as necessary
  
• Walking or moving daily
  
• Drink enough
  
• Avoid tobacco, alcohol and risky substances
  
• Managing stress (breathing exercises, prenatal yoga)
  
• Be well-informed about childbirth and the postpartum period
  

These tips improve daily comfort and gently prepare the body for childbirth.

10. Common errors in pregnancy calculation

Many women are mistaken about some common points:

• Use the date of conception instead of the last menstrual period.
  
• Forgetting that SA and SG do not have the same starting point
  
• Ignoring irregular cycles
  
• Believing that the DPA is 100% accurate
  
• Confusing ovulation symptoms with those of early pregnancy
  

Our calendar automatically corrects these errors to give you a reliable and easy-to-understand estimate.