Over the last decade and accelerating in the wake of the 2022 US Supreme Court decision that struck down Roe v Wade, individual US states have introduced legislation to limit access to abortion. Many of these laws are based on gestational age, which is the standard dating system used to measure the length of a pregnancy in weeks. However, some of these new laws have been proposed or passed based on fetal development markers instead, like detection of heartbeat, ability of fetus to feel pain, and viability of a fetus to survive outside the womb.
Gestational Age vs Fetal Development
Gestational age is the term commonly used to describe how far along a pregnancy is. It is a marker based on the mother’s biology and is measured in weeks from the first day of their last menstrual cycle to the current date. The date of conception generally occurs 2 weeks after the start of gestation. During week 3, a fertilized egg or zygote begins to develop as it travels out of the fallopian tube to the uterus, becoming a blastocyst. Successful implantation in the uterus occurs in week 4, 8 to 10 days after fertilization. A normal pregnancy ranges from 37 to 42 weeks. Gestational age is also broken into trimesters: First Trimester (weeks 1 to 13), Second Trimester (weeks 14 to 27), and Third Trimester (weeks 28 to delivery).
Fetal development is the term used to describe the succession of biological events that occur in pregnancy based on the status of the zygote as it progresses from blastocyst to embryo to fetus to newborn baby. Fetal development is also divided into stages: Germinal Stage (weeks 2 to 4) refers to the weeks after conception when the uterus undergoes changes to accept the blastocyst, Embryonic Stage (weeks 5-9) when the embryo forms most of its organ systems and begins developing bones, and Fetal Stage (weeks 10 to delivery) when the fetus completes these and other stages of development such as gaining external sex organs, fingerprints, teeth, hair.
Fetal development markers are challenging for use clinically or in legislation, because they do not provide a consistently precise way to determine how advanced a pregnancy is. Not all development markers occur at the same time across all pregnancies, and a genetic or physical abnormality or other physical issue may delay or even halt development. And some fetal development markers are the still the object of debate within the medical community.
Interpreting “Fetal Heartbeat,” “Viability,” and “Fetal Pain”
The three fetal development markers used most often in anti-abortion literature and abortion legislation are fetal heartbeat, fetal pain, and fetal viability.
Fetal Heartbeat – This term is often used informally to describe cardiac activity detected in an embryo. This usually occurs around 6 weeks into pregnancy, but it can occur as early as 5 weeks. However, the cardiac tissue in an embryo does not fully develop into a heart until week 10 of pregnancy. A healthcare professional will typically check fetal anatomy including the heart in ultrasounds given between 18 and 22 weeks.
Fetal Viability – The concept of ‘fetal viability’ has been central to abortion legislation in the US and was the standard used in the 1973 Roe v Wade decision. It is generally understood to be the point at which a fetus has the ability to survive outside the uterus on its own or supported by medical technology prior to the expected date of natural birth.
When Roe v Wade was decided in 1973, fetal viability was around 28 weeks or 7 months. With improvements in medical technology, today fetal viability is generally considered to be at 23 to 24 weeks or around 6 months – this is contingent on fetal and maternal health and on external factors like what medical resources are available during delivery.
Viability is a date of potential not definite survivability. Rates of neonatal survival at this point of pregnancy are 23% to 27% for births at 23 weeks, 42% to 59% for births at 24 weeks, and 67% to 76% for births at 25 weeks of gestation (American College of Obstetricians and Gynecologists). For a deeper discussion of viability, see here.
Fetal Pain – There is no medical consensus on when a fetus can feel pain as a sentient being and whether this development occurs before or after viability. Scientific arguments for when fetal perception of pain develops range from 18 to 24 weeks (4 ½ to 6 months), with a few scholars using neuroscience to suggest that a range from 12 to 24 weeks (3 to 6 months) is not impossible.
This article is part of a series on a variety of factors affecting the public perception of abortion in the United States.