Determining Pregnancy

Determining if a doe is pregnant can be a bit of a mystery, particularly if a potential breeding/pregnancy is unplanned. This can be a very anxious time for either a new rat owner or a breeder.
A mucus plug after mating is a sign that the rat has been mated successfully. The mucus plug, or copulatory plug, is formed by secretions from the male rat’s sexual accessory glands. The purpose of the plug is to hold the sperm in the female’s vagina. It may also be used to prevent other males from impregnating the female. Although the presence of a plug does not mean a female definitely is pregnant, it certainly does increase the probability. Sometimes the plug will fall out and you usually, though not always, can find it in the cage

Palpitation of the rat’s abdominal area to feel for fetuses is not recommended. Often they are too small to be felt within the uterus, and the gentlest manipulation may damage them. Palpation is safer after the fetuses are 14 days old, but should still be performed with the utmost care.

Traditional methods for determining pregnancy include:

  • Cessation of estrus cycle
  • Copulatory plug after mating
  • Behavioral changes
  • Tracking weight gain
  • Hair loss around nipples
  • Nest building
  • Enlarged lower abdomen

Cessation of Estrus Cycle

Your female will not have heat cycles after she becomes pregnant. If she skips a cycle it may mean that she was pregnant, but lost the litter. Unfortunately not all female rats have obvious estrus cycles. In this case you will need to watch for other signs of pregnancy.

Behavioral changes

Pregnancy alters the hormonal balance in female rats which can sometimes cause behavioral changes. Your once docile female may become more aggressive and territorial toward both people and other rats. A flighty or aggressive female may become calmer.

Once pregnant, the female rat also shows no interest in males. Handling your males before handling your females usually gets a strong response of sniffing and interest from the females. A pregnant female will not react to the scent of a male.

Your pregnant doe may also show more interest in food and may steal from the other females, be more protective of her food, and stash it.

Often pregnancy behaviors will run in lines.

Tracking Weight Gain

Weighing your female rat prior to mating can give you a good baseline to determine pregnancy weight gain and is the most reliable method for determining and monitoring a pregnancy from the onset.


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 rat. Use a scale that will weigh in grams or ounces, and is accurate to 1 gram, with a maximum weight of no more than 6 pounds. Scales such as these can be purchased at pharmacies, chain stores such as Wal-Mart or Target, as well as online. You will find photos of scales and modified containers to contain rats in the figure section at the bottom of this article.

Patterns of Weight Gain

A female may begin to gain weight within 24 hours after mating. In fact, there may be very few days during an entire pregnancy that the female doesn’t gain weight even if it is only 1/10th of an ounce.
A daily increase gives assurance throughout the pregnancy that babies are growing within. The amount of weight the female will gain will depend on her body mass at onset of the pregnancy, her diet, as well as the number of babies she is carrying.

Once you start weighing your pregnant females you may see a very clear pattern of immediate significant weight gain followed by a period of slow steady gain with a very significant weight gain the last few days.

A typical fetus at full term averages 5-6 grams. A large litter can mean a lot of extra weight on your female. At term females with a decent sized litter can gain as much as 90 grams. Occasionally a rat will carry such a small litter (even one or two) making it almost impossible for the owner to identify a pregnancy until there is birth.

Weight Fluctuations in Pregnancy

While weight gain is used to monitor a healthy pregnancy, it can also be used as a tool to signal a problem. There may be variations in weight during pregnancy such as a slight weight loss, a few days with no weight gain, or bursts of weight gain. These variations are not necessarily abnormal or indicative of a problem.

Time passed with no weight gain or continued weight losses are usually indicative of trouble.
A rat that is obviously pregnant that plateaus at a certain weight, and maintains that weight, raises some serious concerns. It is possible that the fetuses have died without being reabsorbed, the weight gain was due to other medical issues unrelated to pregnancy, or that the babies are stuck and the rat is unable to deliver. If her delivery time has passed without babies being born immediate veterinary intervention is in order.

Consistent weight loss is typically a sign of reabsorption. Often the weight loss will be ½ ounce a day or more. There are many different scenarios that can cause a doe to reabsorb her litter. For the most part the fetuses will be reabsorbed with no problems.

Occasionally incomplete reabsorption may result in a uterine infection. After the loss of a litter monitor your doe for any signs of infection or illness such as abnormal vaginal discharge, high (or lowered) temperature, lethargy, persistent or frank bleeding, loss of appetite, etc. If any of these occur have your doe examined immediately by your veterinarian.

Hair Loss Around Nipples

This sign of pregnancy does not occur until the last week before birth. The loss of hair around the doe’s nipples will make it easier for the new babies to suckle.

Nest Building

As the pregnancy becomes near to term there are increases, and also decreases, of certain hormones that raise maternal behavior in rats. When these changes occur the female begins to build her nest; you can rest assured that babies will be arriving within a few days. Some females make very simple nests while some make elaborate nests that completely hide any view of the babies.

Late in the pregnancy it is a good idea to provide nesting material for the doe. Try to avoid cloth with holes and strings, as these can strangle or injure the babies. You may also want to consider removing hammocks or moving the doe to a one-level cage so as not to cause injury to babies who might fall. (Occasionally females will make their nests and give birth on ledges, in hammocks, or in hanging tubes.)

Enlarged Lower Abdomen

In the final stage of pregnancy, the doe will usually become pear shaped. Her lower abdomen will enlarge significantly and become very firm. This change in her body shape is usually rather dramatic unless her litter is very small.

Photos and Examples of Determining Pregnancy

  • Fig. 1: Mucus plug and semen
  • Fig. 2: Scales and weighing
  • Fig. 3: Pregnancy Weight Chart


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