When do you get ultrasounds during pregnancy third trimester

This guide was originally published on May 15, 2019 in NYT Parenting.

Early pregnancy can feel unreal. How do you know the fetus is there when you can’t see it or feel it moving yet? So the first ultrasound picture can feel momentous: Finally, your whirring brain can be placated with direct proof. But other than the visual evidence that you’re not dreaming all this, you may not know what to expect from prenatal ultrasounds. We’re here to walk you through it.

During an ultrasound exam, your health care provider will record the fetus’s movement, heartbeat, breathing and the amount of amniotic fluid surrounding your baby. But there’s a lot more to know about the tests.

For this guide, I interviewed two obstetricians who specialize in ultrasounds — and a radiologist whose observation led to a diagnostic breakthrough — to find out what you need to know about prenatal ultrasounds.

What to Do

• Prepare yourself for testing.
• Know what to expect at 12-14 weeks.
• Know what to expect at 18-20 weeks.
• Understand that you may need more ultrasounds.
• When to Worry

Prepare yourself for testing.

Usually an ultrasound technician or your ob-gyn will perform an ultrasound. Get ultrasounds at an accredited facility, not at an unaccredited place at the mall. An unaccredited facility “can either miss something or find something that will upset you that’s not really there,” said Dr. Beryl Benacerraf, professor of radiology and of obstetrics, gynecology and reproductive biology at Brigham and Women’s Hospital and Harvard Medical School in Boston. Yes, you will get pictures of the baby, but an ultrasound is not primarily a photo shoot. It’s O.K. if the result is a blurry picture of the baby’s elbow, as long as your health care provider gets the necessary medical information.

It’s common to get a transabdominal ultrasound, meaning that the ultrasound wand, or transducer, is pressed up against the abdomen, lubricated with gel. But it’s possible you will need a transvaginal ultrasound, meaning that the transducer is placed inside your vagina. Transvaginal ultrasounds may be necessary in very early pregnancy, to look at the shape of your uterus, if you’re obese or if your medical team has trouble seeing things with a transabdominal ultrasound. You can ask to insert the transducer yourself if that would make you more comfortable, said Dr. Joshua Copel, a professor of obstetrics and gynecology at Yale, who offers this option.

You’re usually told to drink water before a first trimester transabdominal ultrasound because a full bladder makes it easier to see structures under or behind the bladder. This is not necessary for a transvaginal ultrasound or in late second or third trimester.

Think about who you will want in the room for a medical test. “Sometimes you do find things that are upsetting, and you don’t want to bring the whole family to something that you want to keep private,” Dr. Benacerraf said.

Know what to expect at 12-14 weeks.

The 12-14 week ultrasound scan has four main purposes: determining the due date; detecting multiple pregnancies; checking basic anatomy; and performing a nuchal translucency test, which is a screening test for Down Syndrome and some rarer conditions.

“The earlier you do a scan in pregnancy, the more accuracy it has” for determining your due date, said Dr. Copel. If the 12-14 week ultrasound is your first, it will give the most accurate due date compared to later scans. It’s often more accurate than measuring from your last menstrual period, because dating from the last menstrual period assumes that every woman has 28-day menstrual cycles, and many do not. (Using in-vitro fertilization dates can be more accurate than even very early ultrasounds if you know how many days after fertilization the embryos were transferred. Using records of when you had sex can be accurate, if you kept accurate records and have regular cycles.)

Basic anatomy is visible on the 12-14 week ultrasound. Your medical provider can see if the heart has four chambers; if the brain has two hemispheres; and if the baby has eyes, limbs, lungs, a spine and a bladder, said Dr. Caterina Bilardo, president of the International Society for Ultrasound in Obstetrics and Gynecology. But, she said, “at such an early stage you don’t want to diagnose very subtle heart anomalies.” The full anatomy scan will come later.

The 12-14 week ultrasound includes a screening test for Down syndrome and other chromosome abnormalities called the nuchal translucency test. Nuchal translucency is the thickness of the back of the baby’s neck. The screening test also involves a blood draw, which is used to measure levels of a protein that is lower in Down syndrome pregnancies and a placental hormone that is higher in Down syndrome pregnancies. Dr. Benacerraf was among the first in her field to note that neck thickness on ultrasound was correlated with Down syndrome. “Babies with Down syndrome have a little bit of a fat roll on the back of their neck. Pediatricians have known this for decades,” she said. “We just noticed this in the fetus early on.”

The nuchal translucency test does not tell you whether the baby has Down syndrome, only the chance or probability that he or she does. Even if you get a nuchal translucency test result showing a 90 percent chance of Down Syndrome, you can’t tell whether the baby has Down syndrome without an amniocentesis or chorionic villi sampling test.

Nuchal translucency tests also tell 5 percent of patients that they have a higher risk of fetal chromosome abnormalities when their risk is actually normal, said Dr. Copel.

There’s now another screening test for Down syndrome, called a cell-free DNA test, which involves only a blood draw from the mother, not an ultrasound. “Just because you’ve had the cell-free DNA, do not forgo the ultrasound. They’re not equivalent,” Dr. Benacerraf advised. The cell-free DNA test cannot detect any of the basic anatomy problems that an ultrasound can, such as major heart defects, she said. (Dr. Benacerraf noted that many of her patients who receive the test experience pushback from their insurance companies, which refuse to pay for both the 12-14 week ultrasound and the cell-free DNA test. Check with your insurance provider if you plan to take the blood test in addition to the ultrasound.)

Know what to expect at 18-20 weeks.

The 18-20 week ultrasound is often called the “anatomy scan.” “At that time the organs are already completely formed,” said Dr. Bilardo. Medical providers check for heart defects, make sure all organs are present and count the number of fingers and toes. They also keep an eye on the baby’s growth.

“At that time the organs are already completely formed,” Dr. Bilardo said.

Your provider can to tell you the probable biological sex of the baby at the 18-20 week ultrasound, if you want to know. If you don’t want to know, tell the ultrasound technician or ob-gyn before the ultrasound.

At the 18-20 week ultrasound, your medical provider will also check the position of the placenta. If the placenta is low in the uterus, covering the cervix, this is called placenta previa, and you will need a later ultrasound to see whether the placenta has moved. About 20 percent of patients have “low-lying placentas” at some point, Dr. Benacerraf said, but less than 1 percent end up that way at the end. If the placenta doesn’t migrate and remains resting on your cervix as you approach your due date, you will probably need a C-section, because delivery through the cervix is impeded if the placenta is in the way.

[What to expect from a C-section]

If you had a previous C-section, your medical provider will also check whether the placenta could have implanted within the scar in your uterus from the surgery — a condition called placenta accreta that can necessitate another C-section. If the placenta appears to be on the scar at your 18-20 week ultrasound, you do not necessarily have placenta accreta, because it can be hard to tell from imaging. Your doctor will likely order an additional ultrasound to either rule out or confirm placenta accreta later in your pregnancy.

Understand that you may need more ultrasounds.

An expectant parent might require prenatal ultrasounds beyond the two standard ones for many reasons. Early in pregnancy, bleeding or cramping calls for an ultrasound, both Drs. Copel and Benacerraf said. An early ultrasound can confirm that the pregnancy is in the uterus and not ectopic. An ectopic pregnancy is implanted outside the uterus, and needs to be removed or dissolved because it’s dangerous for the mother and not viable.

Later in pregnancy, ultrasounds may be needed to monitor the baby’s size if there is a risk of too much or too little growth, for example, in some cases of gestational diabetes or pre-eclampsia (high blood pressure during pregnancy). If you have a history of preterm birth, transvaginal ultrasounds can be used later in pregnancy to measure the length of your cervix for early warning signs that you might go into labor prematurely.

If you are expecting twins, you will probably get more ultrasounds to check their growth. In the case of identical twins who share a placenta, Dr. Bilardo said, a pregnant woman will get ultrasounds every two weeks during weeks 16-28 to monitor the babies’ growth and ensure that one of them isn’t hogging an outsize portion of resources from their shared placenta. This condition is called twin-to-twin transfusion syndrome and is treatable either by early delivery or by lasering blood vessels in the placenta.

When to Worry

• If you don’t understand something, it’s always O.K. to ask your medical provider what it means, what your options are or why he or she is recommending a particular follow-up.

• Waiting for test results is hard, and it’s normal for emotions to run high. Seek support from people you trust to be validating and non-judgmental — perhaps other expectant parents or close friends or family.

• It’s normal to feel upset or worried if you get test results saying that your baby has a higher risk for something. Seek out or ask your medical provider to connect you with people who have been in the same situation.

• If your medical provider has trouble explaining results in a way that you understand, you may want to seek a second opinion.

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Anna Nowogrodzki is a science journalist and mom in the Boston area.