Illustration by Marta Pucci
What is the difference between an embryo, a fetus, and a baby?
Embryo, fetus, baby: the stages of development during pregnancy.
Top things to know
The embryonic stage begins with fertilization and lasts for eight weeks
From the 10th week of pregnancy (if you’re counting from the start of the last period) until birth is the fetal stage
A rapid and complex transition happens at birth, from dependent fetus to independent newborn
The language we use to describe pregnancy and the developing embryo or fetus is important. Unfortunately, language has become part of the fight over access to abortion
While you likely hear people talk about the “baby” when someone is pregnant, there are specific terms that describe the different stages of pregnancy. When egg and sperm meet, a zygote is formed and quickly begins dividing to become an embryo. As pregnancy progresses the embryo becomes a fetus. The fetus becomes a neonate or newborn at birth. It may not be common to hear a pregnant person speak about their embryo or fetus in those terms, but we can all choose to use the words we are most comfortable with. A better understanding of embryonic and fetal development can expand our word choices.
How far along am I?
First, a note about how the timing of these different stages of pregnancy are measured. The gestational age is the age (usually in weeks) of the embryo or fetus. When someone says they are 15 weeks pregnant, for example, this is the gestational age of their fetus.
In a clinical setting, like when you see your healthcare provider, gestational age is estimated from the first day of the last menstrual period (LMP) because this is typically an easy date to pinpoint for the pregnant person (1, 2). For embryologists (people who study the development of embryos), gestational age is based on the timing of fertilization (2).
The exact date of fertilization is typically harder to identify (except in cases of assisted reproductive technology, like in vitro fertilization or IVF). Fertilization usually occurs within 12 hours after ovulation (2). The timing of ovulation during the menstrual cycle can vary, but the clinical method of determining gestational age assumes that it occurs on the 14th day of a 28-day cycle (1). Of course, this assumption may be inaccurate because there is a range of typical menstrual cycle lengths and the day of ovulation can fluctuate cycle-to-cycle, even for the same person.
This creates a two-week(ish) discrepancy between the actual age of an embryo or fetus (which begins with fertilization) and how we estimate it based on someone’s last period. In this article, we will be using gestational age based on the time from the last menstrual period meaning you are two weeks pregnant the day you become pregnant, unless otherwise noted.
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What is an embryo?
Fertilization occurs when a sperm cell enters a mature oocyte (egg cell) (2). The DNA from these two cells combine, forming a zygote (2). The zygote repeatedly divides into smaller embryonic cells.
Once the embryo contains 12 to 16 cells, it is called a morula (2). About four days after fertilization the morula nears the uterine cavity (inside of the uterus) it develops a fluid pocket called a blastocele, forming a pocket of fluid surrounded by cells (3). The embryo is now called a blastocyst (2).
About six days after fertilization, the blastocyst typically attaches to the endometrium (uterine lining) and over the next few days burrows through the endometrium so that it can take nutrients from its surroundings (2). Successful pregnancies typically burrow or implant during the implantation window, which is the receptive phase of the endometrium. It occurs 5-6 days after ovulation and closes 3-4 days later (3). By the end of the 3rd week of pregnancy (when counting from the last period), the embryo is receiving nutrients from the pregnant person’s blood supply (2). The embryo (and later the fetus) is reliant on the pregnant person’s blood (which carry oxygen and nutrients) through the placenta. The placenta is an organ specially formed from the blastocyst cell layer called the trophectoderm (3). The placenta is an important organ formed inside the uterus during pregnancy that has several functions, such as bringing nutrients and oxygen to the embryo or fetus and carrying wastes and carbon dioxide away through the umbilical cord (2). The placenta also makes hormones that maintain the pregnancy, influences changes in the body, and provides what the fetus needs to grow and develop (3). The placenta should normally last for the entire pregnancy and will be either pushed out of the uterus with a vaginal birth or removed during a cesarean section when the fetus is born.
The embryonic stage lasts for eight weeks after fertilization occurs (2). This is the same as saying that the embryonic stage lasts until someone is ten weeks pregnant when counting from the start of their last period.
Major milestones during the embryonic stage (2):
Week 3: implantation occurs
Week 4: the beginnings of the central nervous system form
Week 5: cardiac activity begins in what will become the heart, and the eyes, ears, and upper limb buds (arms) begin to form
Week 6: lower limb buds (legs) begin to form, hands and feet start to form
Week 7: fingers appear
Week 8: eyelids form
Week 9: external genitalia begins to differentiate
What is a fetus?
The fetal stage begins at 10 weeks from the last period and lasts until birth (2). By the beginning of this stage, all the major organ systems have formed, but are immature (2). From this point on, the fetus will primarily be growing and tissues will be maturing.
There is no exact timing of fetal “viability” (or ability to survive outside the uterus), but a fetus that is at least 24 weeks may be viable if given intensive care after birth (2). Before 30 weeks gestational age, a fetus is less likely to survive than an older fetus because their lungs and brains are immature (2).
Major milestones during the fetal stage (2):
Weeks 10-13: the fetus is undergoing rapid growth, kidneys begin producing urine
Weeks 14-17: external genitalia has formed, coordinated limb movements, bones are hardening, eye movement begins
Weeks 18-21: eyebrows and head hair are visible, formation of the fetal uterus and vagina
Weeks 22-26: the fetus is gaining weight, fingernails are present
Weeks 27-30: lungs and brain are developed to the point that the fetus would likely survive if born at this point and given intensive care; eyelids are open, toenails are visible, the fetus is putting on fat
Weeks 31-35: pupils respond to light
Weeks 36-40: has a firm grasp, adding 14 grams (about ½ an ounce) of fat per day
What is a newborn?
The transition from fetus to newborn (also called neonate)—which occurs at birth—is complex and must happen quickly for the newborn to become able to survive independently (4). The fetus prepares for the transition by producing hormones (such as cortisol, adrenaline, and thyroid hormones) that will surge at birth, allowing the newborn to quickly begin maintaining normal blood sugar levels, body temperature, and blood pressure (5, 6).
The newborn's cardiovascular and respiratory systems go through an elaborate transition. The fetal circulatory system has extra connections that allow more of the oxygenated blood that it receives through the umbilical cord from the placenta to reach the developing brain and heart and to mostly bypass the lungs. After birth, as the newborn begins to breathe on their own, these extra cardiovascular connections close off (6). Blood flow to the lungs increases as the neonate or newborn must breathe to pick up its own oxygen in the lungs (5).
Milestones in the first year of life
Development continues at a quick pace over the first year of life. Newborns come equipped with reflexes, some of which contribute to their ability to find the nipple, suck and swallow, and survive (5). In the first two months of life, an infant will start to smile, coo and gurgle, turn their head toward sounds, pay attention to faces, and hold their head up when lying on their belly (6). And by the end of the first year, an infant may have favorite things and people, hand you a book to read, say a few words, and be taking steps (7).
Why the words we use are important
The language we use to describe pregnancy and the developing embryo or fetus is important. Unfortunately, language has become part of the fight over access to abortion. Media outlets have put out guidance for their journalists on how to use certain terms when reporting on abortion, noting that language can be manipulated for political gain and not medical accuracy (11, 12).
The language we're exposed to may impact our attitudes toward abortion. In one study, Polish college students took a survey on fetal development. About half of the group was randomly assigned questions that used the term "fetus" and the other half answered questions that used the term "child". The group that was exposed to questions that used the word “child” to describe a fetus was less likely to support abortion when polled at the end of the study (10).
Several so-called “fetal heartbeat” bills were introduced on the state level in the US in 2019 (13). While none of these laws are in effect as of May 1, 2021 (14), this legislation aimed to ban abortion once embryonic cardiac activity can be detected (which is seen earliest on an ultrasound around 6 weeks of pregnancy) (2). Many people may not know they are pregnant this early in the pregnancy and their options are limited by such legislation.
Some people believe that the term "fetal heartbeat" was used by lawmakers not only to elicit a visceral response about “heartbeats,” but also in an attempt to redefine viability by suggesting that embryonic cardiac activity is a sign of viability (15). In reality, the true definition of viability means that a fetus is able to survive outside the uterus and generally isn’t possible for another 18 weeks after detection of embryonic cardiac activity (around 24 weeks of pregnancy) (2), and only then, with intensive care.
The Clue Pregnancy Mode uses medically accurate terminology when referring to embryos, fetuses (feti), and infants, but you can choose to use the language that makes sense for you when talking about your pregnancy. Some people may choose to call their embryo or fetus a “baby” because it helps them feel more bonded, while other people may prefer the more scientific language. Feel free to let people know what terms you would like them to use in reference to your pregnancy.
Upgrade to Clue Plus to track your pregnancy from embryo to fetus to newborn.
If you’re considering abortion and need more information, read our article about what to expect, before, during, and after abortion.
