First trimester of pregnancy: the complete guide from weeks 1 to 13
The first trimester of pregnancy extends from week 1 to week 13 (weeks of gestation) and corresponds to a key phase in the baby's development, from embryo to fetus. For the expectant mother, it is often accompanied by morning sickness , pregnancy fatigue , and hormonal changes. This is also the time for the first medical appointments, the first trimester ultrasound , and the official declaration of pregnancy . Establishing good habits now—such as a suitable diet, prenatal vitamins, and skin hydration—helps lay the foundation for a peaceful pregnancy.
When exactly does the first term begin and end?
The first trimester extends from week 1 to week 13 of pregnancy.
In France, we speak in terms of weeks of amenorrhea (SA) , calculated from the first day of the last period.
Conception usually takes place two weeks later: this is referred to as weeks of pregnancy (WG).
- 13 weeks of gestation (SA) corresponds to approximately 11 weeks of pregnancy (SG).
- The pregnancy declaration must be made before 14 weeks of gestation.
- The due date calculation is done during the first consultation.
Dating ultrasound allows for precise confirmation of the gestational age.
The end of the first trimester is an important milestone: the risk of miscarriage decreases significantly after 12 weeks of gestation, which often brings great relief.
Baby development: from embryo to fetus
Month 1: Fertilization and implantation
Meeting between egg and sperm
-Embryo formation
-Implantation in the uterus
-Production of the hCG hormone
Pregnancy hormones (HCG) increase rapidly, causing the first symptoms.
Month 2: Organogenesis
This is the key phase of development:
-The heart begins to beat
-The members are taking shape
-The brain and vital organs are forming
During this period, folic acid and prenatal vitamins play an essential role in supporting embryonic development.
Month 3: It becomes a fetus and moves
-The organs are formed
-Facial features become more refined
-The first movements appear (still imperceptible)
We are now officially referring to it as a fetus.
At this stage, the risk of miscarriage decreases significantly.
What changes for the mother: Symptoms and upheavals
The first trimester is often intense.
Common symptoms
- Morning sickness
- Significant pregnancy fatigue
- Sore breasts
- Hypersensitivity to smells
- Mood swings
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Nausea Saver
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These symptoms are linked to hormonal variations and the increase in pregnancy hormones.
The growing belly
In the first trimester, the growing belly remains discreet. It's often more a matter of bloating than a true pregnancy belly.
Emotions
Joy can coexist with worry. It's normal to feel vulnerable, especially when faced with the risk of miscarriage. Resting and surrounding yourself with support is essential, and if it reassures you, you can also take your time with simple preparations like packing your hospital bag , without pressure, just to gently envision the future.
Medical follow-up: Essential appointments
It allows:
- To confirm the pregnancy
- To fix the term
- To prescribe blood tests
- To address diet and lifestyle
The pregnancy declaration is then sent to the health organizations.
First trimester (T1) ultrasound
Performed around 12 weeks of gestation, it allows:
- To check the viability of the fetus
- To measure nuchal translucency
- To assess the risk of Down syndrome
Further information is available from official sources such as Ameli.fr or the French National Authority for Health (HAS ).
Taking care of yourself: Good habits to adopt now
Diet and lifestyle
- Avoid raw meat and fish
- Wash fruits and vegetables thoroughly
- Limit caffeine
- Maintain good hydration
Establish a routine from the first quarter
Even if the belly is not yet visible, the skin is beginning to transform under the effect of hormones.
Starting hydration early allows you to:
- To support skin elasticity
- To prevent stretch marks
- To establish a wellness ritual
An anti-stretch mark cream or an anti-stretch mark pregnancy oil can be incorporated from the first weeks, with gentle daily massage.
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Anti-Stretch Mark Ritual
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This moment can become a true time of connection with your body and your baby.
First Quarter FAQs
- How do you calculate the end of the first quarter?
The first trimester ends at 13 weeks of amenorrhea (SA), which is approximately 11 weeks of pregnancy.
- What are the signs of a miscarriage in the first trimester?
Heavy bleeding accompanied by significant pelvic pain requires prompt medical attention.
- Can you feel the baby move in the first trimester?
No, the movements usually become noticeable in the second quarter.
- When should you officially announce your pregnancy?
Many couples wait until the end of the first trimester, when the risk of miscarriage decreases, but this choice remains personal.
