What's Happening Inside Your Body, Week by Week

In the early weeks of the first trimester, the changes inside your body are dramatic — even if you can't feel or see most of them yet. After a fertilized egg travels down the fallopian tube and implants in the uterine lining (around week 3–4), your body begins producing human chorionic gonadotropin (hCG) — the hormone detected by pregnancy tests. Levels of hCG roughly double every 48–72 hours in healthy early pregnancies.

By week 6, a tiny heartbeat may be detectable via ultrasound. By week 10, what began as a cluster of cells has developed into a recognizable embryo with a beating heart, forming limbs, and the beginnings of facial features. By the end of week 12, all major organ systems have been laid down — though they'll continue maturing throughout pregnancy.

For a deeper look at the hormones orchestrating these changes, see how pregnancy hormones affect your body.

Gestational Age vs. Fetal Age

Gestational age (counted from your last menstrual period) is typically about two weeks ahead of actual fetal age. So at "week 6" of pregnancy, the embryo is roughly 4 weeks old. This distinction matters when interpreting ultrasound findings or development milestones. Your provider will use gestational age as the standard throughout your prenatal care.

Common Symptoms and What Causes Them

First trimester symptoms vary considerably between women, but several are especially common:

  • Nausea and vomiting: Driven by rising hCG and estrogen levels. Affects up to 80% of pregnant women and typically peaks around weeks 8–10.
  • Extreme fatigue: Rising progesterone has a sedating effect, and your body is redirecting enormous energy toward fetal development and increased blood production.
  • Breast tenderness: Estrogen and progesterone stimulate breast tissue in preparation for potential nursing.
  • Frequent urination: Your kidneys are filtering more blood, and the growing uterus begins pressing on the bladder.
  • Food aversions and heightened smell sensitivity: Researchers believe these may be evolutionary protective mechanisms, though the exact cause is still studied.
  • Mood changes: Hormonal fluctuations, physical discomfort, and the emotional weight of pregnancy can all affect mood. This is normal, and support is available — see gentle strategies for managing anxiety if you're feeling overwhelmed.

Managing Morning Sickness Day to Day

Many women find that eating small, frequent meals — rather than waiting until hunger is intense — helps reduce nausea. Cold foods, ginger-containing foods, and bland options like crackers are commonly reported as easier to tolerate. If nausea is severe, persistent, or preventing adequate fluid intake, speak with your provider — there are safe interventions available.

Nutrition, Movement, and Early Prenatal Care

The first trimester is a critical window for fetal development, which makes foundational self-care especially meaningful. Folic acid (vitamin B9) is particularly important in these early weeks — adequate intake is associated with reduced risk of neural tube defects. Most prenatal vitamins contain the recommended amount, but always discuss supplementation with your provider before starting anything new.

Gentle movement — like walking or prenatal yoga — is generally well-tolerated in early pregnancy for women without complications, though individual circumstances vary. For general guidance on easing into physical activity, learn what your body needs before starting exercise.

Nutrition during this stage doesn't need to be complicated. Nausea can make eating challenging, and it's okay to prioritize what you can tolerate. Eating well during pregnancy without feeling overwhelmed offers grounded, practical guidance for navigating this.

~80%

Pregnant women experiencing nausea

According to the American College of Obstetricians and Gynecologists (ACOG), nausea and vomiting affect up to 80% of pregnancies.

Weeks 3–4

When implantation typically occurs

Implantation of the fertilized egg in the uterine wall generally takes place between days 6 and 10 after ovulation, corresponding to gestational weeks 3–4.

Week 6

Earliest detectable heartbeat via ultrasound

Cardiac activity in the embryo may first be visible on transvaginal ultrasound as early as gestational week 6, according to clinical obstetric guidelines.

This article is for informational purposes only and is not a substitute for personalized medical advice. Always consult a qualified healthcare provider — such as an OB-GYN or certified midwife — for guidance specific to your pregnancy.