This pain is not sciatica, a very specific pain from an unhappy nerve — unless she has a significant back or trauma history.

Usually, it’s a common, easily fixed problem within the sacroiliac joint  representing a lack of motion — usually on only one side.

If the sacroiliac joint is moving normally, this back pain doesn't occur. 

The sacroiliac joint receives an underwhelming amount of attention from orthopedists.  Hopefully that will change when technology lets us look more easily at the joint. For now, hands-on evaluation of joint motion is the major source of assessment. Usual finding? The joint  is “stuck”. 

When normal motion is restored... (easily and usually in one treatment)... the pain is GONE. Injections offer only a temporary fix. Testing the joint for motion and then restoring normal motion is the only valid option. 

The usual scenario: the woman begins to complain of sharp back pain in her sixth month of pregnancy — at 3 or 4 months in subsequent pregnancies. 

She has no history of back problems. The sharp pain can be totally disabling. Rolling over in bed, stepping off a curb, or walking on an uneven surface typically trigger the pain.

Sacroiliac dysfunction can also cause pain in the hip joint or groin, loss of balance, or R upper quadrant belly pain.

Distress worsens as the pregnancy progresses.

Send her off to see an orthopedic physical therapist, or have a look yourself… here’s some background. 

The sacrum mates with an ilium… "mountains" of the ilium fit into the "valleys" of the sacrum. Normally and with every step, the ilia glide downward. As an ilium moves, the sacrum moves too…tiny movements. 2-6mm.  If there has been a compressive incident in childhood or during a previous labor and delivery, only one ilium may be moving. The immovable side grips the sacrum so that neither the ilium nor the sacrum moves on that side. Either side may be painful.

The evaluative test is usually called THE STEP TEST. 
The client stands, resting one index finger at her side on a wall or table. Make sure she is just "touching down”; she should bear just ounces of  weight on her finger.  Sit on a stool behind her with your thumbs on the bony Posterior Superior Iliac Spine on each side. (The dimple is the inferior spine — you need to be about an inch and a half higher.)  If you have to guess about where to place your thumbs, (the woman is obese), make sure you are wide enough to be on the ilium and not on the sacrum; you want to perceive whether or not the ilium moves down as she lifts her knee just a little bit toward her chest. Your thumbs merely go along for the ride. 

Have her lift her right knee slowly toward her chest … three inches or less and then slowly put her foot back down.  Repeat on the left side. Watch what your thumb is doing. You should see it move down. 

If the joint is stuck, you may see and feel no movement, or you may feel your thumb RISE UP; the ilium isn't moving down as it should! 

RESTORING MOTION. There is a safe, easy way to help her. She will be lying down with the non-moving side UP. Put a nice pillow under her head and under her tummy — a fat pillow between her knees. The underneath leg should be bent for support; her top leg straight.  Her top hip must be right over the bottom hip. She RELAXES.

Stand at the foot of the table, hold her top leg (above the ankle) softly with your hands. Turn her leg a little bit so that her big toe points down.  Now, lean back with your whole body — not just your upper body — and "lengthen" her leg. Do this movement slowly.  Ask her to tell you about increased pain.  If there is pain, gently and slowly stop. If she says that what you are doing feels good and helps alleviate her pain, continue. After a few seconds, ease off and return the leg to its resting position. Repeat this cycle several times. 

Help her sit up, get down and walk to lubricate the newly moving joint. She can ice her back if there is any discomfort — but discomfort is unusual. Movement of any and all kinds will help the joint retain its newfound normalcy. Prepare for her relief, amazement, and gratitude. 

Caution: the stretch to the sacroiliac joint will travel through the knee and hip joint so be sure to ask her if she has any knee or hip problems. If she has any, this technique should not be used. If you don't feel comfortable or have time to try this yourself, locate an orthopedic physical therapist or chiropractor who treats pregnant women and sacroiliac joints.They of course have many more nuances of knowledge.  This article describes the often amazingly helpful “nuts and bolts”.