Believe it or not, your baby has been practicing its breathing since the end of the first trimester! Of course the baby is not really breathing yet, it is surrounded by fluid and does not ‘breathe’ the fluid—it is only pushing the fluid in and out of its mouth and throat. The baby gets its oxygen from the mother who supplies fully oxygenated blood to the baby via the placenta and the umbilical cord. But like a little athlete, the baby is in training and is not just lying there, passively growing. It has important jobs to do to get ready to live outside of the womb. It needs to practice moving the diaphragm up and down and exercising those breathing muscles so when it comes time to take that first breath on the big day, it is ready. It’s also practicing sucking, swallowing, and moving its body.
One side-effect of all that practicing—hiccups! The diaphragm and nervous system are still very immature so often, a good bout of practice breathing and swallowing will result in a strong case of hiccups. Hiccups start early, as early as 12 weeks. Mothers often will feel hiccups and be very confused by the rhythmic thump-thump that they feel. It’s slower than the heartbeat (although some new moms will confuse it for that) but regular and rhythmic, lasting anywhere from a minute to much longer. Don’t worry, it isn’t the baby’s first case of hiccups and it won’t be the last. The baby doesn’t mind or find it bothersome, and you should take it as a reassuring sign that your baby is doing its homework and working those muscles.
The breathing motions (also known as practice breathing or fetal respirations) start early and should increase in frequency and duration as the pregnancy progresses. Observing fetal breathing movements with ultrasound is important part of the biophysical profile and is a sign that the baby is doing well. Below is a video of a 33 week old fetus in 2D that is very busy breathing, hiccuping, sucking, and swallowing. The heart can be seen beating in the chest, while the rhytmic up/down motions of the thorax are the breathing movements. You can see the hiccups by the sudden big jumps, and the baby’s mouth and tongue can be seen working on swallowing and sucking. The black is the amniotic fluid, and the gray area above the black is placenta. Click for more information on understanding your ultrasound.
Even if you see that the baby is ‘breathing’ a lot, it is not associated with lung maturity. By 37-38 weeks, almost all babies lungs are mature enough to adequately oxygenate the baby’s blood after it is born, but there is no way to tell for sure by just looking at the baby with ultrasound. Fetal breathing movements do NOT mean the baby’s lungs are ready! The only way to determine lung maturity with certainly is to do an amniocentesis to draw a sample of amniotic fluid from around the baby and do a chemical analysis. This is only done in specific cases when doctors have reason to want to get the baby out of the womb early, but can wait until the lungs are mature.