In dogs, there are two primary measurements which you can perform to estimate foetal age. In this context, the term gestational age refers to the number of days following the luteinising hormone (LH) surge with the average length of gestation being 65 days.
The first method involves measuring the diameter of the gestational sac and is more accurate in early gestation (before Day 40).
The second method of estimating foetal age involves measuring the diameter of the foetal head. This method should be used to estimate the foetal age in late gestation (after Day 40).
The formulas for these methods are (all measurements are in centimetres) :
Gestational age = (6 x gestational sac diameter) + 20
Gestational age = (15 x head diameter) + 20
Fig 1: Ultrasound image of a canine foetus demonstrating measurement of head diameter.
Therefore, the days remaining prior to parturition can be determined by subtracting the calculated gestational age from 65.
Foetal growth occurs in a linear fashion from Days 17-30 and subsequently becomes exponential. In other words, small breeds will grow more slowly and giant breeds grow more quickly after Day 30.
However, foetal growth in dams whose non-pregnant bodyweight is in the range of 9-40kg is less variable.
Therefore, a few points to remember when ageing the canine foetus using ultrasound:
Ageing the Feline Foetus
For ageing the feline foetus, the value for the average length of gestation is 61 days. As with the canine foetus, both gestational sac diameter and head diameter measurements may be used when estimating the age of kittens.
Before Day 30 (measurement in millimetres):
Gestational age = (1.0901 x gestational sac diameter) – 0.9372
After Day 30 (measurements in centimetres):
Gestational age = (25 x head diameter) + 3
(GSD). As the gestational sac is not always perfectly round, two measurements are taken and the average value for GSD is used in the formula for calculating gestational age.
Assessing Foetal Viability
Ultrasound imaging is also useful for identifying foetal distress or death. Movement of the foetus and observation of a foetal heartbeat (with or without the use of colour Doppler) are parameters which cannot be identified on a radiograph.
The foetal heart rate is generally twice that of the dam or queen. A reduction in foetal heart rate can indicate that the puppy or kitten is in distress, such as occurs with hypoxia secondary to dystocia.
Fig 3: Ultrasound image of a feline foetus demonstrating bloodflow through the heart using colour Doppler. The heartbeat can also be readily visualised in the ‘live’ black and white image without the use of Doppler.
Kutzler, MA, Yeager, AE, Mohammed, HO and Meyers-Wallen, VN (2003). Accuracy of canine parturition date prediction using fetal measurements obtained by ultrasonography. Theriogenology 60:1309-1317.
Nyland, TG and Mattoon, JS (1995). Veterinary diagnostic ultrasound. W.B. Saunders Company, Philadelphia.
O’Brien, R and Barr, F (Eds) (2009). BSAVA Manual of canine and feline abdominal imaging. British Small Animal Veterinary Association, Gloucester.