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The Biology of Pregnancy Part 3: Late Embryogenesis

During the 6th week of pregnancy —meaning the 6th week since the first day of the most recent menstrual period— a woman often notices that something is different, other than the fact that she has missed her period. In particular, this is the time when pregnancy induced nausea and vomiting —commonly called ‘morning sickness’, although it can occur at any time of day— sets in, although it can begin a little earlier too. Nausea and vomiting symptoms may group worse from this point at peak at about nine weeks into pregnancy. For many new mothers, these symptoms are the first clue that lead to a pregnancy test and/or a trip to the doctor.

Meanwhile, inside the uterus, by this time, the embryo is the size of a pea, and you can make out the difference between body, head, and tail on ultrasound. The developing heart and lungs are moving down into what will become the chest cavity. Inside the heart, the atrium is separating noticeably into left and right chamber. The liver and kidneys are emerging too, plus tiny limb buds are forming that will become the baby’s arms and legs. Another notable feature on ultrasound that you may see at your obstetric exam is the placenta.

Closure of the neural tube that we discussed in Part 2 is just about complete by the end of week six. A central nervous system (consisting of a brain and a spinal cord) is emerging. However, deficiency of folate can lead to neural tube defects –gaps in the tissues that line and protect the brain or spinal cord. Other risk factors for neural tube defects include diabetes, obesity, and certain anti-seizure medications.

Smoking, including second hand smoke, is also a risk factor for neural tube defects, and many other birth defects too. Although studies suggest that low levels of tobacco exposure, even levels associated with second hand smoke are associated with preterm birth, evidence suggests that the risk is the product of repeated exposures. Thus, if you smoked once, your baby is probably not at elevated risk, even if that one episode of smoking occurred six weeks into pregnancy, a very critical point in development. On the other hand, if you smoked repeatedly (a more common scenario), the risk would be significant, even if that repeated smoking took place prior to pregnancy. Binge smoking has not been studied much in pregnant women, but this behavior can result in carbon monoxide poisoning and nicotine toxicity, disrupting oxygen delivery to the fetus and blood supply of the womb, respectively, putting you at severe risk of early pregnancy loss. Risk of sudden infant death syndrome and long-term effects is elevated for maternal smoking occurring at any point in the pregnancy.

If you haven’t seen your obstetrician yet, it’s important that you make a visit. The embryo is particularly vulnerable to permanent damage from agents such as alcohol, hundreds of substances that are present in tobacco smoke, and nutritional deficiencies, especially inadequate folate. It is vital that you have a checkup to see whether the embryo is growing normally, and to discuss whether you need to make any lifestyle changes.

In the meantime, keep in mind that you should not be smoking at all. Nor should you be drinking more than a very modest amount, and let’s be clear. Modest means a few sips of wine or beer per day, not a glass or two.

At a gestational age of seven weeks, various organ systems within the embryo are coming online or just about ready to do so. The lips and tooth buds are beginning to form. Bile ducts of the embryonic liver are becoming visible. Within a few weeks, during the fetal period, the liver will be producing red blood cells, but for now those cells are generated in the yolk sac. Closer to the end of pregnancy, that function will move again, this time to the bone marrow, which also is under development already by week seven. Meanwhile, the liver will gradually gain capability for numerous tasks. The kidneys are on deck to begin filtering out waste from the blood. By seven weeks into pregnancy, the embryonic heart has three chambers, like the heart of an amphibian, but the single ventricle has begun to separate into two separate chambers. Your obstetrician will be looking for a heart rate that is at least 120 beats per minute.

You will notice changes in the baby’s head in particular. Eyes and ears are now barely recognizable. As for the size, from crown to rump, the embryo now measures roughly 11-13 millimeters. At this point, seven weeks into pregnancy, there is still a realistic chance that the pregnancy will not make it to term. The most common complication at this point in pregnancy is pregnancy loss, what doctors call spontaneous abortion. But spontaneous abortion becomes far less likely once you pass this point. In the meantime, moderate exercise is recommended –not skydiving, boxing, no or anything with high risk of body impact, but really most other activities are permitted, including running and cycling.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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