There are many opinions about when an embryo turns into a fetus, with some people feeling that it’s at the point of conception and others saying it’s when you can hear a fetal heartbeat. In reality, changes are continually happening hour-by-hour and day-by-day. So when does an embryo turn into a fetus, officially? Medical terminology puts the end of the embryonic period and beginning of the fetal period at week 10, according to the National Institutes of Health.
During pregnancy, your baby is reaching new growth milestones every day from conception. In fact, your pregnancy begins even before that, since your expected delivery date (EDD) is calculated from the first day of your last period, It counts as part of your 40-week pregnancy, although your baby hasn’t even been conceived yet. Gestational age (or menstrual age) is the time measured from the first day of your last menstrual period to the present date, in weeks.
How does the fetus develop during the first trimester?
The first trimester is about 3 months or weeks 1 – 12.
Upon conception, the egg is fertilized by the sperm, becoming a zygote. After a few days of cell division, it becomes a blastocyst, reaching the womb (uterus) around day 5 and implanting itself into the uterine wall on about day 6.
The embryo: Twelve days after conception, placental blood circulation and hormone production begins, often detectable in the pregnant woman’s urine by a home pregnancy test. The embryonic stage is where differentiation begins: the creation of cell types that become blood cells, kidney and nerve cells. By 5 weeks the heart has begun to beat and the central nervous system to form, plus tiny buds appear which will be the arms and legs of the now peanut-shaped embryo.
Over the next few weeks up to week 10, fingers and toes form, but may still be webbed. The brain and lungs, elbows and toes, eyelids, ears, facial features and all essential bodily organs have begun to form. Around the end of the first trimester, the baby can make a fist and the head makes up about half of the baby’s size.
What happens in the 2nd and 3rd Trimesters?
The second trimester is week 13 – 27 and the third trimester is from week 28 to birth. In the second trimester, during weeks 15 to 18, fetal movements begin. In weeks 19 – 21, the baby can hear and you may feel fluttering in your lower abdomen. At week 22, you will begin to feel the baby moving and a fetal heartbeat can be heard with a stethoscope. The baby develops a startle reflex at about week 26 and rapid brain development occurs as you move into the third trimester. In that final trimester, the nervous and respiratory systems develop, bones begin to form and fingernails and hair grow.
How can I follow my baby’s growth in detail?
In order to understand in detail how your baby is developing between prenatal doctor visits, refer to the Pregnancy Calendar. For personalized information about your pregnancy and fetal development, join BabyQ. You can also take the BabyQ Survey to improve your BabyQ.