The best week for a 4D ultrasound, in weeks and months

Weeks and months: how the stretches convert

Pregnancy is counted in weeks from the first day of your last period, not from conception, and Cleveland Clinic puts a full-term pregnancy at 40 weeks, or 280 days (Cleveland Clinic). How far along am I? explains the counting.

Months are rougher. Cleveland Clinic’s guide counts four weeks to a month: “if you’re three months pregnant, you’re about 12 weeks.” On its chart:

• 16 weeks: the end of month 4 (weeks 13 to 16).
• 17 to 24 weeks: months 5 and 6.
• 25 to 28 weeks: month 7.
• 29 to 32 weeks: month 8.
• 33 to 36 weeks: month 9.
• 37 to 40 weeks: month 10.

So the whole-baby stretch runs from the end of month 4 to the end of month 6, and the face stretch from the end of month 7 into month 9. Calendar months average a little more than four weeks. Depending on the months and the week you deliver, Cleveland Clinic says, “you could be pregnant for either nine months or 10 months.” Treat any month as approximate, and book by weeks.

What a 4D scan shows, stretch by stretch

We group the weeks into three stretches from what we see in our studio. That’s our experience, not a study’s finding; the sources under each stretch explain what’s changing. Weeks 25 to 27 sit between the first two, and a 4D session is open any week from 16 to 40.

16 to 24 weeks: the whole baby

“Your baby is small enough to fit in one frame,” as our 4D page puts it. “Arms, legs, a full stretch, the occasional somersault.” The NHS puts a baby at about 11.6 cm (4½ inches) from head to bottom at 16 weeks, and 30 cm (12 inches) from head to heel at 24 weeks (NHS, NHS).

By 20 weeks, the NHS describes “kicking, punching and turning around” (NHS). Cleveland Clinic says that in month 4, a baby can already suck a thumb, yawn, stretch and make faces. Those early faces aren’t expressions yet: at 16 weeks, the NHS says, any smiling or frowning “will be completely random, as there’s no muscle control yet.”

Cleveland Clinic describes thin skin at 17 weeks, just starting to put on fat, and in month 6 skin that is “reddish in color, wrinkled,” with veins showing through. So a 3D still from these weeks shows a leaner face than you’ll see later. From about week 25, more body fat makes the skin “less wrinkled and plumper,” it says.

This stretch also holds the medical anatomy scan: at about 18 to 22 weeks in the U.S. (Cleveland Clinic), and usually 18 to 21 weeks in England (NHS). A keepsake 4D session doesn’t check your baby’s anatomy, growth or health, and it doesn’t replace that scan.

28 to 34 weeks: the face

“Cheeks have filled out, and the whole body no longer fits in the picture,” our 4D page says. “This is the time for close-ups of the face and hands.” The NHS puts your baby at about 37.6 cm (15 inches) from head to heel at 28 weeks, and 42.4 cm (16½ inches) at 32 (NHS, NHS).

One study measured that change in face shape, though not which week gives the clearest picture. Researchers from London and Parma, Italy, recruited 235 women having scans at two specialist fetal medicine centers, and modeled 3D face scans of 135 babies at 24 to 34 weeks. In its model of the average face, later weeks were “associated with wider fuller cheeks, flatter foreheads, deeper eyes and slightly bigger noses” (Sivera and colleagues, 2024). At 32 weeks, the NHS says, your baby “needs to put on weight,” and the “extra chubbiness” helps them keep warm after birth.

Faces also do more. A team led from Durham University filmed 15 healthy babies in 4D at about 24, 28, 32 and 36 weeks, after recruiting their mothers at a hospital in Middlesbrough, England. Their facial expressions grew “more complex” with the weeks (Reissland and colleagues, 2013). Yawns went the other way. In the team’s paper on 15 babies scanned at the same weeks, the average count in 10 minutes of footage fell from about 2 at 24 weeks to none at 36 (Reissland and colleagues, 2012). These are small studies, and no week promises a yawn.

35 weeks to your due date: still a good time

“A bigger baby, less fluid and less room, so the view depends on how your baby is lying,” our 4D page says. “We work around the position and take our time.” Here’s what medical sources say about each part:

• A bigger baby. About 47.4 cm (18½ inches) from head to heel at 36 weeks, and 51.2 cm (20 inches) at 40, the NHS says.
• Less fluid, nearer the due date. Cleveland Clinic says fluid levels “peak at 34 to 36 weeks and then slowly decrease as you reach your due date” (Cleveland Clinic).
• Less room. The NHS says there will be “less space in your womb” as your baby grows, and by 40 weeks your baby is “getting rather squashed up” (NHS, NHS).
• A settled position. By 36 weeks your baby “may already have moved head down into your pelvis.” By 37, “around 95% of babies will now be head down” (NHS, NHS).

What affects the view at any week

Some things matter at every week, and no one can choose them in advance.

The 2026 London study is a reminder that a good face isn’t guaranteed: in its research scans, 49% of women got at least one good or moderate 3D face image. Whether the baby was head down, the fluid, the week and BMI weren’t significantly linked to quality there, but the sample was small.

Is it too late at 36, 37 or 38 weeks?

Not at our studio: 4D runs from 16 weeks to your due date, and from 35 weeks it’s “still a good time.” The view depends on how your baby is lying. We can’t promise a perfect face on a given day, and nobody can. Plan for three things:

• Your baby may arrive first. At 37 weeks, the NHS says, “Your baby could come any day now – and this would not be considered early” (NHS). In the U.S., ACOG and the Society for Maternal-Fetal Medicine call 37 weeks to 38 weeks 6 days “early term,” and 39 weeks to 40 weeks 6 days “full term” (NICHD). If a late look matters to you, book it nearer 35 weeks than 40.
• Plans can move the date. If your doctor or midwife has planned an induction or a cesarean, count back from that date, not your due date.
• Movements don’t wind down. Less room doesn’t mean less moving: the NHS says you should feel your baby move “right up to and during labour” (NHS).

Twins: does the timing change?

The weeks count the same way, but twins change the calendar.

• Many twins arrive before 37 weeks. NICE, which writes health guidelines in England, says “about 60 in 100 twin pregnancies result in spontaneous birth before 37 weeks.” For uncomplicated twins, it recommends offering a planned birth at 37 weeks if each twin has its own placenta. If they share one, it’s 36 weeks, or 32 to 33 weeks if they also share a sac (NICE). Can you see twins at 6 weeks? explains twin types.
• Twins reach for each other early. In an Italian study, five low-risk twin pregnancies were filmed in 4D at 14 and 18 weeks, at the Institute of Child Health in Trieste, and the hand movements were measured later, frame by frame. The twins made movements “specifically aimed at the co-twin,” and by 18 weeks these were 29% of the movements measured (Castiello and colleagues, 2010). It’s a research finding from five pairs, not a promise for any session.

So with twins, the last stretch can be short, or never come. Ask your doctor or midwife when your babies are likely to be born, and pick your week from that. For 4D with twins at our studio, book as usual and tell us when you book, so we plan the time.

One visit or two: picking your week

• The whole baby, moving: 16 to 24 weeks.
• A fuller face, close up: 28 to 34 weeks. If you want one visit and you want the face, aim for 28 to 34 weeks.
• Both: some families come twice, once early for the whole baby and once later for the face.
• Boy or girl as well: from 16 weeks, a 4D session includes it, if you’d like to know.
• 35 weeks to your due date: still a good time, though the view depends on how your baby is lying. It’s never too late.
• 3D stills: the same weeks. 3D is a still picture with depth, and 4D is that picture moving, live (2D, 3D or 4D?).

Whatever the week, a 4D scan is an ultrasound. The FDA says ultrasound has “an excellent safety record,” but can heat tissue slightly, and “The long-term consequences of these effects are still unknown.” The FDA and the AIUM discourage scans done only for keepsakes (FDA, AIUM). ACOG says it should be used “prudently and only when use is expected to answer a relevant clinical question or otherwise provide medical benefit to the patient” (ACOG). What is and isn’t known is in 4D ultrasound side effects and Is an early ultrasound safe for the baby?, and Is a 4D ultrasound worth it? weighs the value.

In weeks, where are you today? Type your due date into the planner to find out, and to see which sessions fit this week and which come up next. The date stays on your device.

4D in Glendale, from 16 weeks to your due date

Our 4D Bonding session runs from 16 to 40 weeks, at our studio in Glendale, Los Angeles County, open since 2006. There’s no single best week, because early and late sessions show you different things, and it’s never too late. Every 4D session here gives you both 3D and 4D: you watch in 4D, and take stills home.

It includes live 4D on the big screen, 100+ color images on a USB drive, 12 color prints, and boy or girl, if you’d like to know. Every session books online. Same-day appointments are often available. See every session and price.

When a session is particularly difficult, we take extra time, and we bring you back for another try. It’s a keepsake visit, not a medical exam (what an elective ultrasound is): it doesn’t check your baby’s anatomy, growth or health, or replace any scan your doctor or midwife orders. On bonding, the AIUM finds “only some scientific evidence that 3D sonography has a positive impact on parental-fetal bonding” (what the bonding research says). Come for the joy of it, not for a health benefit. See the 4D session, find your week, or read Safety, in plain words.

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