The Biology of Pregnancy Part 15: Organ Systems Prepare for Birth

When gestational week 35 rolls around, you have just five weeks to go until delivery, if you have a singleton, and it could be a little sooner. If you have twins, your due date is coming in just about one week. Average fetal weight at this point is 2,380 grams (5.25 pounds). Plenty of babies are born with this weight at term, but from now to birth the weight of your fetus will increase by more than 40 percent.

As for the lungs, they are are usually mature by this point. They’re boosting their production of surfactant, the soapy substance that enables the lungs to expand. Surfactant contains chemicals called phospholipids and the ratio of the types of those phospholipids in the surfactant is approaching what it should be in the lungs of a newborn infant. For this reason, in the event of premature delivery at 35 weeks, the survival rate is very high.

As for the cardiovascular system, it’s not entirely ready for life outside the womb. In fetal life, there is a connection between two big blood vessels that exit the heart: the aorta and the pulmonary artery. Known as the ductus arteriosus, we have discussed it many times here on The Pulse. Because pressure is high in the pulmonary artery and low in the aorta, the ductus arteriosus diverts blood away from the deflated, fetal lungs, into the aorta.

But once the baby is born and takes its first breath, pressure in blood vessels of the lungs decreases, while pressure in the left side of the heart and in the aorta rises. Blood from the right side of the heart then can travel through the pulmonary artery to the lungs to receive oxygen, and the ductus arteriosus usually closes within a couple of days, if not sooner. It then transforms into a ligament. Closure of the ductus arteriosus can be delayed in a premature infant, but doctors can administer an NSAID drug, such as indomethacin, or ibuprofen, to accelerate closure of the ductus arteriosus. For the same reason, pregnant women must not take NSAIDs during late pregnancy, since that can cause premature closure of the ductus arteriosus, meaning closure while the fetus is still in the womb, which would be devastating. On the other hand, there are situations in which it is good to have the ductus arteriosus remain open for a long time after birth, namely certain types of congenital heart disease, when often there is a need to find an alternate pathway for getting blood to the lungs. In such cases, maintaining the ductus arteriosus open, which is achieved by administering medicines called prostaglandins, often starting before birth, will allow blood to move from the aorta through the ductus arteriosus into the pulmonary artery in the newborn. This is the opposite direction in which the ductus arteriosus channels blood in the fetus.

As for the eyes, the 35th week is a time when the fetal pupils react normally to light. They dilate, if bright light is shined through the mother’s pelvis. They constrict when the light is removed. Kidneys are fully developed by this point too, while the liver is nearly up to speed with its many functions. But space is getting very limited inside the womb, so the volume of amniotic fluid continues to drop and kicks actually start to subside. To the mother, they may feel more like rolls and shakes.

Now, the longer that the fetus stays in the womb, the later the delivery date, the less time the baby tends to need to stay in the hospital after delivery. For those born at 36 weeks, the average time in hospital is 11 days, compared with 51 days for those born at 30 weeks and 163 days for an infant born at 23 weeks. Also, survival is much better for later premature births than for early ones. As for long-term effects, problems with health and cognition are fairly common for very premature infants, but more rare, and usually less severe, for those born at 36-38 weeks. And by the way, infants born from 38 weeks onward are not even considered premature.

At this point, particularly for first time mothers, the fetus may descend deep into the pelvic region. This is called lightening and it predicts that delivery will occur in roughly two to four weeks. If you have already given birth, however, the time between lightening and delivery can be much shorter. When the lightening process begins, pressure on your ribcage decreases. This provides more space for your diaphragm to move up and down as you breathe, which can be a relief. On the other hand, since the fetus is lower, now there may be even more pressure on your bladder than there was before. As you can imagine, this will increase the need to pee frequently.

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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