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It’s common for women to have a 7 week ultrasound and for many, it’s their first real look at their baby. A dating scan at 7 weeks helps to confirm baby’s presence and development, but it can also help to rule out pregnancy complications.
At , the embryo matures to a foetus and individual genetics and growth factors begin to influence growth. An early pregnancy dating scan, when compared with scans later in pregnancy, can be more accurate when assessing the expected date of delivery. With maturity, the size of the baby correlates less to its age than in the early weeks
Your embryo will be measured from the top of its head, the “crown” to its bottom or “rump”. This is called the crown-rump-length or CRL. This is because it is the longest portion of the baby’s body gives an ideal measurement of its growth and development.
An average length of the embryo at 7 weeks is between 5-9 mm long. The average weight is less than 1 gram. Obviously, every pregnancy is unique and individual factors influence the size of the embryo.
There are two ways of having a seven week ultrasound. One is via the abdomen – transabdominally and the other is through the vagina – transvaginally. At 7 weeks gestation, transvaginal ultrasound provides the best and most accurate visualisation. This is because the transducer does not need to send sound waves through multiple layers of muscle and tissue to “pick up” the returning images.
When a transvaginal ultrasound is done at seven weeks gestation, the transducer is placed in the mother’s vagina and the sound waves are transmitted via her cervix directly into the uterus. With this form of scan it is not as necessary for the mother to have a full bladder. When having a transabdominal ultrasound, however, a partially full bladder is recommended. This helps to move the bowel out from the pelvis into the abdomen so the uterus, ovaries and pregnancy can be seen more clearly. Later on as the pregnancy progresses, a full bladder is not necessary as the enlarging uterus is no longer contained in the pelvic rim.
In very early pregnancy, the embryo and pregnancy sac may simply be too small to see very much at all. It’s the shape and general structures which are more obvious. And if present, a tiny heart beat. The average number of beats per minute (BPM) is between 100-180 between 6-7 weeks of gestation.
At seven weeks of gestation it is still too early to identify what gender (sex) the baby will be. It is also impossible to do a thorough foetal screening assessment because it is still just too premature in terms of embryonic development. However, general “mass” structures such as a head and body can generally be seen in the embryo at seven weeks.
Yes, one of the primary reasons for a dating scan at 7 weeks is to count how many gestational sacs and embryos are present.
If possible, this is a nice experience to share. For many partners, this is the first time the pregnancy seems ‘real’.
At the 7 weeks can, only a gestational sac and yolk sac may be seen. It’s still very early in the pregnancy. If there are concerns, you may be asked to return for another scan in 7-10 days to check on the embryo’s development.
Written and reviewed by Jane Barry, midwife and child health nurse on 6/01/20.