American Fancy Rat & Mouse Association

This article is from the WSSF 2004 AFRMA Rat & Mouse Tales news-magazine.

“How can I tell if my female is pregnant?”

By Carol Lawton

This is one of the most common questions from new breeders and has always been a bit of a mystery. The traditional method includes checking for a mucous plug a few hours after mating, watching for behavioral changes, then waiting two weeks to see if the female gets round. For those of us who are new to breeding, this can be a very anxious time.

The issue was compounded for me by wanting to be certain that my babies, when they finally arrived, were normal. The only data available was from lab sites, and lab data doesn’t always apply to our pets. Since I was planning to breed my lovely RI Coco Chanel for the second time, I started looking for a scale to weigh her babies.

The scale most suitable for this is a small digital kitchen model with either a flat surface or a removable bowl that is large enough to hold an adult male rat. Try to find one that will weigh in grams or ounces and is accurate to 1 gram, with a maximum weight of no more than 6 pounds. Target and Wal-mart have affordable models, but eBay has the best variety. I eventually found a small digital kitchen scale with a flat surface that weighs up to four pounds in either tenths of an ounce or grams. It seemed to be perfect for my rattery. I suddenly found myself weighing everything in sight and before long, all of my rats were accustomed to perching on the scale for their daily weigh in. But things took a different turn when my lovely RI Coco Chanel became pregnant.

I was stunned to discover that Chanel gained weight in the first 24 hours after mating, in fact there were very few days during her entire pregnancy that she did not gain at least one-tenth of an ounce. This daily increase was a wonderful assurance throughout her pregnancy there was no doubt she was progressing normally. Chanel produced seven beautiful healthy 8 gram babies right on time.

After my experience with Chanel, I began monitoring all of my pregnant girls. After a few litters I noticed a very clear pattern, immediate significant weight gain, a period of slow steady gain, and very significant weight gain the last few days.

These are the charts for Nayda and Kendra—they show typical gain during the first week. Nayda is a slightly smaller girl and gained 1.2 ounces, or 11% of her normal weight, Kendra gained 1.4 ounces, 11.7% of her normal weight. See Table 1.

Table 1. 1st Week

Weight in
Day 111.212.2
Day 211.412.6
Day 311.612.9
Day 411.813.3
Day 512.013.3
Day 612.113.5
Day 712.113.3

As their pregnancies progressed you can see that there is a slight gain almost daily. Nayda gained a total of six-tenths of an ounce, Kendra gained nine-tenths of an ounce. See Table 2.

Table 2. 2nd Week

Weight in  
Day 812.013.6
Day 912.213.7
Day 1012.313.8
Day 1112.413.9
Day 1212.514.0
Day 1312.614.2
Day 1412.814.2
Day 1512.614.2

As the girls went into their third week I had a slight scare with Nayda–her weight dropped slightly causing me to be concerned about the pregnancy. She regained the weight over the next few days and her pregnancy seemed to progress, but her abnormally low weight gain was worrisome. Kendra’s pregnancy is fairly typical with a gain of more than half an ounce some days. See Table 3.

Table 3. 3rd Week

Weight in  
Day 1612.614.2
Day 1712.614.4
Day 1812.714.7
Day 1913.014.9
Day 2013.015.5
Day 2113.216.0
Day 2213.316.5
Day 2313.113.2

Kendra delivered 11 lovely babies 22 days and 4 hours after breeding. All of the babies were 6 to 7 grams each with a total litter weight of 71 grams or 2.5 ounces (28.35 grams per ounce). By adding in 1–2 grams per baby of birthing product, and a slight blood loss for mom, it comes very close to Kendra’s post-birthing weight loss of 3.3 ounces. Kendra’s weight increased steadily over the next week and she settled at a nursing weight of 14.7 ounces.

There doesn’t seem to be a typical total weight gain for a pregnant rat, much depends on the size of the mother, her diet, the number in the litter, the size of the individual babies, and the amount of fluid the female retains for milk production. But one thing is very clear in all but the smallest of litters: a pregnant rat will gain weight almost every day.

Nayda’s slight weight loss at the end of the second week and her abnormal gain during the last week were obvious signs of a problem. Nayda never showed signs of nesting or labor. Her weight dropped over the next seven days and she was returned to her normal happy life with the other spoiled girls.

While weight gain is a wonderful way to monitor a healthy pregnancy, it can also be used as a tool to signal a problem. A significant weight loss during pregnancy–more than one-half ounce in a single day–is typically a sign that the litter is being reabsorbed. While Nayda did not exhibit a significant weight loss until after her due date, her minimal weight gain was enough to cause concern, and it was no surprise that she didn’t deliver. When I’m certain that a female has reabsorbed, I like to watch her weight closely until she has returned to her pre-pregnancy weight and resumed her normal hormone cycle. A weight gain while reabsorbing could be a sign of infection or abnormal fluid build up and veterinary assistance should be sought.

Weighing babies is a fun excuse to spend more time with them–it can also offer peace of mind that the smaller babies are indeed thriving. I prefer to monitor their weight in grams since it’s more accurate (2.835 grams = 1/10 ounce). This is the chart for Kendra’s litter–it shows the weights for the smallest and largest of the litter. Their birth weight was 6 to 7 grams. See Table 4.

Table 4.

1 Day
7–8 g
8 Days
14–19 g
15 Days
29–38 g
2 Days
8–9 g
9 Days
15–21 g
16 Days
32–42 g
3 Days
9–10 g
10 Days
17–22 g
17 Days
35–43 g
4 Days
10–12 g
11 Days
20–25 g
18 Days
37–47 g
5 Days
11–14 g
12 Days
22–28 g
19 Days
40–51 g
6 Days
12–16 g
13 Days
25–32 g
20 Days
43–55 g
7 Days
13–17 g
14 Days
27–34 g
21 Days
46–61 g

A typical healthy baby from an average litter is around 6 to 7 grams at birth, babies born at 5 grams are considered small, and those over 8 grams are large. Unless you have a marked or very diverse litter, it’s not easy to track a single baby’s weight gain from birth, but daily spot checks on the largest, smallest, and someone in between is a very easy and enjoyable task. But be careful . . . as a long-time breeder recently discovered, weighing your babies can be addicting. *

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Updated March 20, 2014