Reproductive Biology for Politicians and Voters, Part 1

Embryos, IVF, and fetal development

After the results of the 2024 election, it seems that over the next few years, politicians in the USA will seek to pass legislation regulating various issues of human reproduction and developmental biology. My goal in writing this post is to explain the scientific facts that are relevant to these issues. Of course I have my own political views, but here, I will avoid passing judgement on what ought to be the law, so that hopefully legislators and voters of any party can use these facts to make well-informed decisions.

The first part of this guide will focus on embryos and fetal development, and the second part will focus on sex development, puberty, and transgender-related issues.

Embryos and in vitro fertilization

Does a human life begin at conception? The answer depends on what we consider to be a human life. However, scientifically speaking, embryos do not have a 1-to-1 correspondence with people.

1. Most human embryos die before developing to a baby. The fraction varies, with the age of the mother’s egg being the most important factor, but even in couples with peak fertility, more than half of naturally conceived embryos die.

2. Up until the developmental stage of gastrulation (discussed below), it’s possible for an embryo to split in two, forming identical twin humans. There have even been rare cases of identical triplets. Therefore an embryo does not correspond to a unique human being.¹

3. A pre-implantation embryo has no more sensory awareness than a skin cell or liver cell.

Diagram of early human embryonic development. In IVF, embryos are typically cultured for about five days, until they reach the blastocyst stage (#6) before transfer into the uterus. The typical diameter of a blastocyst is 140 µm, or about 6 times smaller than a poppyseed. Source: OpenStax Anatomy and Physiology (CC-BY)

For many infertile couples, in vitro fertilization (IVF) is the only way to have biological children. IVF involves collecting eggs from a woman’s ovaries, adding sperm to fertilize them, and then transferring the resulting embryos (ideally one at a time) to a woman’s uterus² until a pregnancy is established. The number of embryos created during a cycle of IVF is limited by the number of eggs retrieved, which depends on the woman’s age but is typically about 10-20. This is roughly as many embryos as a couple will produce in 1-2 years of trying to conceive naturally (at a rate of 1 embryo per 28 days).

Timeline of human pregnancy

In medical practice, pregnancy is measured in gestational weeks, the number of weeks counting from the date of the woman’s last menstrual period.³ For a woman with typical fertility, ovulation occurs two weeks into the menstrual cycle, so the time of fertilization is already gestational week two. The earliest sign of pregnancy is typically a missed period.⁴ Even in ideal circumstances, this is not possible to notice until an additional two weeks (gestational week four). Since many women have irregular menstrual cycles, they might not realize they are pregnant until gestational week five, six, or even later.

Key developmental milestones include:

• Gastrulation happens at gestational week 4–5 (14–21 days post fertilization).⁵ This is the developmental process that creates different lineages of embryonic cells that will go on to form different organs. After gastrulation it is no longer possible for the embryo to split and form identical twins.⁶

• Heart formation happens around gestational week 6. The heart is the first organ to form in the fetus, so detection of a fetal heartbeat does not necessarily indicate that the brain or other organs have developed. At this stage the embryo is about 3 mm (roughly the size of a peppercorn). For images see here.

• Brain formation begins around gestational week 7 but the activity of the brain at this stage is still quite limited. The earliest plausible stage at which a fetus could feel pain is gestational week 12 although some researchers have argued this capacity does not develop until gestational week 24. Although it is impossible to ask a fetus whether it is actually experiencing pain, the biological structures required for pain response are definitely established by week 24, and some aspects of these structures begin to form earlier (around week 12).

• Viability of the fetus outside the uterus depends on the level of medical technology. There are current examples of fetuses as young as 21.5 gestational weeks surviving premature birth. However, babies born this premature have a low survival rate, require prolonged stays in a neonatal intensive care unit, and generally have severe long-term issues.

• Full-term birth happens at gestational week 38-40 (anything earlier than week 37 is considered preterm).

• For a more detailed overview of developmental stages (including pictures) I recommend the Carnegie collection.

In the USA as of 2021, 45% of abortions were performed at ≤6 weeks of gestation, 81% were performed at ≤9 weeks of gestation, and nearly all (93.5%) were performed at ≤13 weeks of gestation.

About 15% of all recognized pregnancies end in miscarriage (this statistic excludes early miscarriages that happen before a woman realizes she is pregnant). When a woman has a miscarriage, she may need medical help to remove the fetal tissue from her uterus so that it does not cause complications such as infection.

Ectopic pregnancies can occur when an embryo implants in a location outside of the uterus. This happens in about 2% of pregnancies and requires medical removal of the fetus to prevent life-threatening complications such as bleeding. An ectopic pregnancy cannot result in a live birth.

Final thoughts

Obviously, questions of IVF and abortion are very politically inflammatory. But political opinions don’t change scientific facts – and if legislators or voters don’t understand the facts, they may not realize the consequences of proposed legislation. For example, banning abortion after a fetal heartbeat is detectable is banning it at a much earlier developmental stage than most people realize.

If any politicians (from any party) would like scientific advice on these issues, or have a question about something I haven’t covered here, I’d be happy to help. You can reach me at metacelsus [at] protonmail.com

1. Importantly, religions which believe that humans have souls generally hold that these souls are indivisible. In Judaism, the embryo is not believed to have a soul until 40 days after conception (gestational week 7.5).

2. When a gestational surrogate is used, the embryo is implanted into a different woman than the one the egg came from.

3. Doctors measure things in gestational weeks, but developmental biologists typically measure in weeks post fertilization. To convert to gestational weeks, add two.

4. A pregnancy test can detect pregnancy as early as 8 days after conception (gestational week 3), but the rate of false negatives at this stage is high, and most women won’t realize they should take a pregnancy test unless they miss their period.

5. In many jurisdictions, culture of human embryos for research purposes is allowed only up to day 14 post-fertilization, which is prior to gastrulation. However the International Society for Stem Cell Research recommends that “Should broad public support be achieved within a jurisdiction, and if local policies and regulations permit, a specialized scientific and ethical oversight process could weigh whether the scientific objectives necessitate and justify the time in culture beyond 14 days, ensuring that only a minimal number of embryos are used to achieve the research objectives.”

6. If a split occurs during gastrulation the result is conjoined twins (or, more likely, simply a miscarriage).