Pregnancy calendar: I trimester

I trimester

Each pregnancy is different. Every child develops at his or her own speed. Our pregnancy calendar tries to give a general overview of how the pregnancy is usually progressing. Caution should be exercised when talking about gestational age and embryonic age of the fetus. Pregnancy weeks are not counted from the moment of conception, but from the first day of mother’s last menstrual period. Therefore, the number of pregnancy weeks and actual embryonic age differ around two weeks.

There are around 40 weeks between the first day of the last menstrual period and childbirth. That time is divided into three trimesters, each lasting around three months. The first trimester is considered to last around 12 to 15 pregnancy weeks, according to different sources.

Prenatal development occurs in three stages:

  • Conception
  • Embryonic Development
  • Fetal Development (starting from 11th pregnancy week (embryonic age approximately 9 weeks) to birth)

The first prenatal checkup to a gynecologist or midwife should occur after the pregnancy test was positive. It is important to make sure that the first appointment is scheduled before week 12, because some tests and screenings must be carried out at certain times. During the first trimester you could go to a doctor at least twice.


An oocyte is a female gametocyte or germ cell, produced in the ovary during female gametogenesis, that stores all the woman’s genetic information.

A spermatozoon or sperm cell is a male gamete, that stores all the man’s genetic information.

A zygote (from Greek ζυγωτός zygōtos “joined” or “yoked”), is the first cell of a new human being, formed when two gamete cells (female and male) are joined by means of sexual reproduction.

An embryo (from Greek ἔμβρυον (embruon), lit. “young one”) – a developing human organism during the first 8 weeks (10 weeks from gestation). All the major organ systems will develop during the embryonic development.

A fetus (Latin fētus, “offspring”) is a developing human organism from 9 weeks (11 weeks from gestation) until birth. During the fetal period, the human body grows at a very fast pace, and his or her major organ systems will start to operate.