If you posted that positive pregnancy test on FB, you know what the first question you are going to get its: Do you have an exact due date?
Your family and buddies will go for the predicted due date and just have fun betting, but for mom and dad: they want a few more facts! What is an exact due date, and how do you know if your due date is right is a common question for the gyno!
Once that due date gets close you have more questions on that exact timing. It's not just if the baby is big enough, as many of the babies get big before they get mature. You're still waiting, and wondering, and you feel you are 'more than ready' to birth that baby. But is the baby ready, just what is happening in that third trimester. The baby’s physiology matures quickly. The heart rate slows from an average of about 180 beats per minute, when we can first measure it, to an average closer to 140 beats per minute close to birth.
The amount of amniotic fluid cannot always predict due date. As the baby grows it’s kidneys can make more urine. By the end of pregnancy the baby is producing over a liter of fluid a day, and this is the main source of the amniotic fluid. Babies gain most of their actual weight in the third trimester.
Size cannot always predict due date. Babies that do not grow are considered growth restricted. Based on the parents ethnic background growth tables for the fetus can be used to determine the relative size of the baby compared to other fetuses of the same number of weeks. Babies that are smaller than the tenth percentile are considered “SGA” Small for Gestational Age. Babies that are heavier than the 90th percentile are considered “LGA” or Large for Gestational Age. Causes of small babies can be genetic problems, nutritional problems, placental problems or hypertension. Large babies are often those of diabetic moms. The most accurate time to measure these size and weight changes is at the beginning of the third trimester. Ultrasound estimates of the weight of the baby become less accurate as the baby gets near to term and may be off by as much as ten percent of the estimated weight. In fact an experienced obstetrician or midwife can probably guess the baby’s weight by examination almost as accurately in most cases!
Many of these questions can be answered by ultrasound, so sit back and enjoy that last one wave from the uterus, it's probably going to be a fun one!